g io i - - ( o w t e ) m JAN 2 61990 SYMPOSIUM ON QUANTUM ELECTRONICS
UNIVERSITY OF POONA
JANUARY 1-3. 1981
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D. D, Bhawalkar U. K- Chatterjee Mrs. N. Y. Mehendale Convener Secretary Local Secretary BARC, Bombay. BARC, Bombay. Univ. of Poona, Pune-
G. Chakrapani B. Ghosh V. V. Itagi I IT, Kanpur, BARC, Bombay, Marathwada Univ. Aurangabad.
J. P. Mittal A. S. Nigvekar U. Nundy BARC, Bombay. Univ. of Poona, Pune. BARC, Bombay.
B. S. Rate! M. K. Raghvendra Rao N. Ramaswamy DSL, Delhi. BARC, Bombay. DAE, Bombay. SYMPOSIUM ON QUANTUM ELECTRONICS UNIVERSITY OF POONA
JANUARY 1-3, 1981
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GROUPS PAGES
I . LASERS 1-go
I I . QUANTUM OPTICS 91-118
I I I . NOHLDIEAH OPTICS 119-150
17. LASEB PHOTO-CHBHISTEY 15U166
V. LASEB SPECTROSCOPY 167-196
7 1 . LASEB SCATTERING 197-212
T H i LASER PRODUCED PLASMAS 213-2)2
,7111. LASEB APPLICATIONS 233-26) LASERS
Water Dielectric coaaxial cstole driven 1 multigas laser system. - V.V.Itagi and A.H.Khan.
An ultraviolet pumped Nitrogen laser * with a water dielectric capacitor. - S.Thattey.
Spectral output of Np lasers at 337.1 ran 8 - C.Lal and S.N.Thakur.
A symmetry in the laser output from 11 transversely excited N2 la9er - C .lal, J.P.Singh and S.N.Thakur.
Analysis of Nj-laser with triggered and 15 untriggered spark-gap. - S.Thattey*
Polarization properties of internal 19 m irro r multimode He-Be la s e r - D.Sen, F.N.Puntambekar, H.S.Dakiys and V.T.Chitnis.
Excitation tremsfer by atom-atom and 22 electron-atom collisions among excited states of Neon. - V.D.Dandawate.
Time resolved studies in pulsed Argon 26 ion laser - V.Hari, C.Chakrapani, P.B.K.Sarma,^ P.B. Bao and P. Venkateshwarln.
Intensity distribution in Excimer 50 transitions of NeE, Arl’ and KrP. - T .T .I ta g i and B .S.B halekar.
Effect of Tripropylamine in ballast 55 resistance type TEA C02 lasers. - (J.K .Chatterjee, U.Nundy, A.K.Nath and N. 3.3 hik arkhane.
M ultiline injection locking in TEA GO^ 57 laser. - A.K.Nath and (J.K.Chatterjee.
Discrepancy of 0- concentration in the 41 dissociation products of a sealed TEA C02 laser - D.J.Biswas and G.K.Chatterjee 13. Problems of Simultaneous triggering of 45 two coaxial plasma tubes in a CW CO^ la s e r . -3.L.Gupta, B.S.Narayan and l.M.Kukreja. 14. Some new laser dyes - M.B.Padhye, 48 T.3 .Varadarajan and A.V.Deshpande.
15. Optical gain in the Ranipal-S laser 56 Dye - V .V .Itagi and B.H.Pawar.
16. Effect of Hydrogen bonding on DAMC 59 Dye laser characteristics. - V.Masilamani and B.M.Sivaram.
17. Efficiency of energy transfer Hh-6G- 63 Hh-3 Dye Laser - P.J.Sebastian and X .Sathianandan.
18. Solvent effect on amplified spontaneous 66 emission of Anthranilic acid - S.Y.Itagi and A.Kulkarni.
19. Antistokes fluorescence in Neutral Bed 70 ex c ite d by 632.8 nm He-Ne la s e r . - S.Y.Itagi and A.Kulkaml.
20. Spectral evolution of a laser pump 74 grating tuned Dye Laser - K.Dasgupta and L.G.Nair.
21. A double pulsed Ruby laser for pulsed 78 holography - R.Chari, G.Chakrapani, Bh.A.R.B.Raju, K.R.Sarma and P.Venkateswarlu.
22. Study of thermal lensing in a high power 82 NdtGlass amplifier using a wavefront shearing interferometer. - L.J.Dhareshwar, T.P.3.Nathan, J.S.Uppal and 9.L.Gupta.
23. Transient lens measurement in optically 86 pumped glass am plifier ▼ T.P.3.Nathan, J.S.Oppal, L.J.Dhareshwar, B.S.Narayan and D.D.Bhawalkar. WATER DIELECTRIC COAXlAi CABLE DRIVER MULTIGAS LASER StSTEM.
V .V . Itagi and A-R- Khan Department of Physics Maratbwada University Aurangabad 431004, India
In recent years substantial efforts have gone into the development of excimer lasers because of their . potentiality of high power .in the ultraviolet region of the spectrum. These lasers have provided experi mentalists with a new and versatile tool for spectro scopic and photochemical investigations. Pumping of these excimers, in particular, rare-gas halide excimers has.been accomplished through (a)high energy pulsed electron beams (b)electron beam sustained discharges and (c)ultraviolet preionized avalanche discharges.
Though higher efficiencies have been achieved in electron beam pumping, the laser pumped by fast transverse discharge is a convenient laboratory instru ment of small size. Discharges of this type have been used in the past for pulsed N 2 and CO? lasers. Many different configurations have been used for the. power supply. These include discrete capacitors1»2', Blumlein circuits with strip transmission lineso»4 and charged coaxial cables'. These networks use solid dielectrics and suffer from dielectric fatigue and breakdown. This difficulty has been overcome by using a water dielectric B lu m le in S ,
Pure water has high electrical breakdown strength.for'pulse" loading of the order of a micro second or less' and is self healing. , since water possess an exceptionally high dielectric constant V € = 80) throughout a broad frequency range,, it permits the fabrication of a low-impedance transmission line with very compact physical dimensiongL We have combined these Useful properties of water and certain advantages of- coaxial, cable discharge in the laser design under report.■ The; power supply to the laser head consists of a parallel array of charged water dielectric coaxial cables.. Each cable is a one foot long aluminium tube with a copper coated m.s. welding rod mounted along the tube axis. Deionized water is made to flow in the intervening space. These cables directly terminate into the laser electrodes. Both the electrodes have chaiig profile® and' are 30cms. long. The anode is a. solid., metal bar and the cathode is made 2 hollow and is covered with monel mesh with an ultra-.- violet preionizer system behind- it. The preionizer is driven by a separate cable. The laser body is all metal and hence can withstand pressures of several atmospheres. The laser head, the electrodes and the preioni zer system are all demountable. So also are the windows and the- reflector mounts. These arrangements help to run the laser with different gases at required pressures along with the necessary optics that goes with each type of laser.
Since water has to be loaded with short duration pulses a two stage Marx generator has been used to supply power to the system through pulse shaping pressurized; switch. The schematic of cable pulse- forming network is shown in fig .2.
The details of the laser system and of the preliminary investigations performed with N 2 a s l a s e r gas will be presented.
We are grateful to the Atomic Energy Commission of India for the financial support
REFERENCES 1. R.Burnham, N . w . H arris, and N.Djeu, Appl.Phvs.Lett. 28, 86(1926). 2. R.Burnham and N.Djeu, Appl.Phys.Lett.29,/0 /(19/6). 3. B.Godard and M.Vannier, Opt.Commun.187206(19/6). 4 • C .P.Christensen, L.W.Braverman, W .H.gteier, and G -W ittig. A P P l.P h y s .L ett. 29,424(19/6) • 5 . R-C-Sze and P.B.Scott. J.Appl.Phys. 4/,5492(19/6). 6. J .I.Levatter and R.s.Bradford, Jr.Appl.Phys.Left. 33, 242(19/8). /. D.B.Fenneman and :R.J .Gripshover Digest of Technical Papers, Second I.E-F.E. Pulsed Power Conference p. 122 (19/9)'. ' 8. T.Y.Chang, Rev.Sci.Instr. 44, 405(19/3)• - 3 -
■ f r e t o ni ^ e r
-Mesli colhode -Win d o u-i -Ar-io -Water- cdi M, Fig. 1. End v iew s c h e m a tic o j . the- X -aser-. S w itc h " ^ L a e e r - E l 'ig.3. . 5 c hem ati c o j , cable PFU . - 4 - AN DMBAVIOIEl POISED NITROGEN LASER WITH A WATER DIEIECTSIC CAPACITOR O.O. BHAUALKAR, Sudhir THATTEY, Pradeep.AGRAUAL, Swapan CHATTERIEE Bhabha Atomic Research Centre Bombay-400 085 The Nitrogen laser is an important source of pulsed UV radiation for pumping dye lasers, photochemistry, spectroscopy etc. The laser can be fabricated in a moderately equipped laboratory. Due to the short life time of the upper laser level, theexcitation to this laser has to be very fast, typically within 10 nsec. This requires ultra-loui—inductance capacitors for storing the electrical energy. Ida report here the fabrication of a. Nitrogen laser which uses a water dielectric flat plate transmission line. Water as a d ie le c tr ic has two main advantages, namely a very high dielectric constant (~ 80) and a high pulsed dielectric strength (~ 1 MU/cm). Thus in a given volume, water dielectric can store a considerably larger amount of energy than, say the fibre glass epoxy laminate. Further, water being a liquid is self healing. The conductivity even of distilled or deionized water is sufficiently high to discharge the energy stored in the dielectric in a few micro-seconds. It is, therefore, necessary to pulse charge the water dielectric, and transfer the energy to the laser all within. a microsecond. The la s e r schem atically shown in f ig . 1 c o n sists of three 1 cm thick square aluminium plates with each side 30 cm long. These were spaced and insulated with perspex. Thickness of water dielectric was 1 cm. It formed a Blumlein circuit with nitrogen laser discharge gap replacing the load. An arrangement was made to flow water but flowing of water was found not necessary. A self fired spark gap was placed a t one end of the ca p a c ito r as shown in f i g . .1 and the nitrogen laser channel was fixed at other end. The central plate was charged to high voltage and the outer ones were grounded, thus minimizing high voltage hazards. Capacitance of the water dielectric flat pla.te line was 25.44 nF, the characteristic impedance was 0.70£1 and delay was.. 17.38 ns. An oil filled Mylar-dielectric' capacitor C1 (fig. 2) was first charged through a charging resistance to the desired voltage and was then discharged through a triggered spark gap 51 (and an inductor to limit the current) into the water dielectric capacitor. This in turn was, within a microsecond,_ discharged through another spark gap S^, into the laser channel. Thus the water dielectric capacitor was kept charged for only about a microsecond. The oil filled capacitor. C1 and water dielectric .capacitor C2, with an inductor formed an 'LC' resonant circuit and the. period of o s c illa tio n was such (0.9 > j s ) that before the spark gap could extinguish the current started flowing in opposite directions This reducsd the effective voltage across the channel and reduced the laser output power* This problem was solved by using a diode chain in series with the Inductor which prevented reversal of current. To measure tha capacitance and resistance of water dielectric capacitor the decay time of the voltage on the capacitor without firing of the spark gap S 2 > was observed using a high-voltage probe and CRO. The decay time was measured to be 5 >13. Further, damped oscillations of 0.9 >13 period were observed when diodes 0j were removed. From these observations the resistance between two elec tro d es of the c a p a c ito r was c a lc u la te d to be 200 £ 1 giving conductivity of the,-double distilled water to be 5 > j siemens. The capacitance was a ls o c a lc u la te d and came out to be 25 hF, which agrees well with that calculated from geometrical considerations. The laser pulse shape was observed with a fast CRO and a niplaner photodiode. The band width of tha CRO limited rise time was I r.ssc. The laser pulse had a FUHH of 7 nsec and full width at base II nsec. The peak power was 150 Kid for upto 25 pps, with a voltage ■c: 17.5 XU on the transmission line. The operating pressure in the l.vser channel was 55 torrs and the spark gap $ 2 ( s e lf - f ir e d ) was a t 1 2 -i. ii'-mospberes. The peak power varies as V as expected. The laser undergone more than 1 million shots and is still working J&- W// 7 /A V7/7///// . TRIGGER i / / /\ ALUMINIUM PERSPEX WATER FIG. 1. NITROGEN LASER WITH WATER DIELECTRIC CAPACITOR D1 SI D2 ■ w - -vwv DO- c sa so n r— 1<3— 30K /1 15uH o - 230 VAC, § KKV RMS -Cl C2 _ '30nF 25nF" (WATER DIELECTRIC) FIG. 2. CIRCUIJS USED FOR CHARGING THE WATER □ ELECTRIC CAPACI TOR * j t o in FIG.3- TYPICAL LA$£R PULSE SPECTRAL OUTPUT OF LASERS AT 3 3 7 .1 nM C.LAL and S.N.THAKUR, Laser and Spectroscopy Laboratory, Department of Physics, Banaras Hindu University, Varanasi-221005 Three types of ^ lasers were used in the present investigation and the laser outputs were photographed in the third order of a 35 ft. concave grating spectrograph with a resolution of 0 .0 5 cm”^ .in order to detect the variation in the intensity of rotational transitions. Two of the lasers were transversely excited using Blumlein circuitry of which one had an inter electrode separation of 1 mm and could work in the pressure range of ICO torr to 700 torr . (l) the . other had an inter-electrode separation of 10 mm and could work in the pressure range of 25 torr to 80 torr (2). The third laser was an axially excited one and could operate a pressure of U torr only (3). The applied voltage for the low pressure transversely excited laser was 15 KV, for the high pressure laser it was 13 KV and for the axially exci ted laser it was 50 KV. All the lasers were used with gas flowing through the system. The axially excited laser output was rather weak and it required 5x10^ laser shots to record tne spectrum whereas for the transversely excited lasers 50 to 100 shots were found' adequate on Kodak spec trum analysis No. 1 photographic plates. The spectral bandwidth of the laser radiation is about 0.1 nm and we. observe the following significant f e a t u r e s : (1) With Ng gas alone in.the laser plasma cavity the num ber of rotational lines decreases with increase in the gas pressure. (2) With a mixture of Ng and argon in the laser plasma cavity there is a large decrease in the intensities of branch lines as compared to the and P 2 b ra n c h l i n e s . (3) When the partial pressure of Ar is more than that of Pj branch lines are completely absent. - g _ (4) At the same total pressure of the gas the rotational lines are sharper in the laser, output from a mixture o f h'2 and Ar than in that from gas alone. (5) In the axially excited laser the rotational lines belonging to P^, and P^ branches appear and their relative intensities are similar to those of the transversely excited lasers at low gas pressure. The operating conditions of the axially excited laser are rather critical such that it does not lase for gas pressure in excess of 4-5 torr. It was thus not possible to carry out measurements in the presence of argon in this c a s e . We have found that the total power output of the lasers decrease at higher working pressures but relative intensities of P^ branch lines decrease faster than those of P^ and P^ branch lines. This suggests that the three com ponents (C-^ ^and c \ Q) of the excited electronic state are all deactivated via non-radiative channels at higher pressure. This observation is in disagreement with that of Ishchenko et al (4) who account for .the decrease in,intensities of P^ branch lines in terms of a competi tion with lasing in the P^•branch lines. The disappearance of P, branch lines at higher gas ■> 3 pressures suggests that the laser action from the C rr0 3 3 ^ component stops earlier than from and C tt0 l e v e l s . This may either be due to a more rapid collisional de- 3 excitation from the C n 2 level or due to a decrease of population below the threshold value for the laser eWiSS on. It is found that argon is more effective in stopf>rflg 3 laser transitions from the Crnw, level since at the same total pressure the P^ lines are missing from the mixture of and Ar but not- from ^ gas alone. This fact along- with the broadening of rotational lines at higher pressu res of Ng gas indicate that in the presence of gas alone there is a non-radiative energy transfer from an - 10 excited N^ molecule to an unexcited one by means of co lli sions . In the presence of argon, however, the excited N2 molecules transfer their internal energy into translation al energy of argon atoms. The present experiments therefore lead to the follo wing conclusions : (1) The K2 laser output at higher pressures shows varia tions in intensity of the rotational transitions that can be accounted in terms of non-radiative de excitations. (2) The pressure of argon may be suitably adjusted to obtain laser transitions from the component level alone. REFERENCES (1) J.P.Singh and S.N.Thakur, J.Res. and Industry, 2J, 227 (1978). (2‘) J.F.Singh, Ph.D. Thesis, Banaras Hindu University, (1 9 8 0 ). £3) C.Lai, J.P.Singh and S.N.Thakur (To be published). (4) -V.N.Ishchenko, V.N.Lisitsin and P.L.Chapovskii, . Opt. and Scectrosc.. 33. 196 (1972). - 11 = ASYI'iMETRY IN THE LASER OUTPUT FROM TRANSVERSELY EXCITED LASER C.LAL, J.P.SINGH and S.N.THAKUR, Laser land Spectroscopy Laboratory, Department of Physics, iBanaras Hindu U niversity, Varanasi-221005. The Blumlein type transversely excited lasers used in our studies have the two electrodes separated from one another horizontally. The intensity distribution along the,horizontal direction across the output beam have b een m easured w ith a EG and G, SGD-04QA p h o to d io d e an d . philips oscilloscope. The photodiode, was kept at a dista nce of 1 meter from the output, end of the laser and the beam was attenuated with a. 12 mm thick transparent perspex sheet before falling on the photodiode. The photodiode is placed on a mount which can be moved horizontally as well as vertifically and can be positioned to within an accuracy of 1 mm. We have used two different laser cavities in our experiment. The first one has an electrode separation of 15 mm with two symmetrical electrodes having rounded convex faces with 3 mm radii of curvature. The second laser has an inter-electrode spacing of 10 mm and one of the electrodes, is sim ilar to that of previous laser, but the other one has a shart edge with a thickness of about 1 am. 'Em fixed applied voltage was 13 KV* The anode is defined as the electrode which is at the positive potential with respect to the other at the time of initiation of discharge in laser cavity,. The upper copper sheets on both sides of the laser channel of the Blumlein capacitor w ill have positive polarity before the breakdown of the spark gap but once it breaks down the copper sheet on tne spark gap side w ill be momentarily at negative potential with respect to the other end of the laser cavity. The anode and the cathode are represented by letters A and C for this situation, the polarity is, however changed if we reverse the connections from the D.C. power supply to the laser system. - 12 - The laser intensity distribution in the case of symmetrical electrodes is found to be asymmetric with its maximum shifted towards the anode. The measured intensity distribution along the horizontal diameter of the laser beam in the case of unsymmetrical electrodes with the sharp edge as cathode is shown in Fig.l. We get two peaks in this case one closer to anode and the other closer to the cathode. A change in the pressure of leads to a shift in the position of the peak closer to the anode whereas • the other one remains unaffected. The observations corre sponding to the situation when sharp edge electrode is at positive potential are shown in Fig.2. In this case only one peak results in the intensity distribution and it is found to shift with variations in gas pressure. The asymmetry in the intensity distribution may be explained, in terms of an asymmetry in the regions of population inversion within the laser channel.. The. obser vations indicate that in the case of a laser channel with symmetrical - electrodes there is only one region of maximum population inversion whereas in the case of asymmetrical electrodes there are two such regions which may result due to space charge build up (l). There are two major causes of a build up of electrode sheaths . (1) Glabal current conservation (2) the. differente in diffusion.and mobility speeds of electrons (fast; and ions (slow). In the case of unsymmetrical electrodes the space charge accumulation is expected to be the greatest near the pointed electrode. The intensified field near a spike enhances ion bombardment and secondary electron emission which results in electron acceleration and subsequent enhancement in molecular excitation. It can be shown (2) that there are two such regions when the sharp edge is cathode whereas only one region when the sharp, edge is - an o d e . - 13 - In the case of tv/c symmetrical eleccrodes the dV region of maximum S — exists only near the anode and •hence there results only one peak in the intensity d istri bution. The change in mobility of electrons at higher gas’ pressures results in a shift in the position of the region of maximum population inversion closer to the anode.. This observation is in qualitative agreement with the measure ments of current pulse in the laser plasma by Anderson and Tobin (3). R e fe re n c e s 1. . A.I-l.Howatson, An Introduction to Gas Discharge, Fergamon Press (1965). 2. L.B'.Loeb, Basic Processes of Gaseous Electronics, University of California Press,. Berkeley and Los Angeles,(1955)• 3. " H.E.B.Anderson and R.C.Tobin, Physica Scripta, 9,.7 (1979). - . O'.-O "TO TORI Mg. ■©—EJ - S® t o **. H i. 3 a Tor * Xg, lw - t o o So 3o 5o d ( in wir) — ♦ Fr6. 2. ** IIH ««"i) FTOx 1 . - 15 - Analysis of T3^ - laser with triggered and untrjgsered a-park gap. Sudhir THATTEY M.D.B.Se, Hxyeics Group BJL.R.C., Bombay-400085* A number of designs of Ng-laser use a f l a t transm ission lin e » and a spark gap to energize the laser plasma. Several excimer lasers also use a similar configuration where the transmission line is re placed by lumped capacitors. In this paper the power requirements of Ng-laser are analysed in the two modes of operation of spark gap, namely triggered and untriggered and the advantage of using a spark gap triggered at appropriate time is illustrated. Analysis of the untriggered s-park gap : In the untriggered case the spark gap self fires at a voltage determined by the gas composition , pressure and separation of the gap. In order to limit the current after the spark gap has self fired, a limiting resistance is incorporated in the circuit. At low frequences the circuit is equivalent to a diode, a resistance and a capacitor in series with a voltage generation Vmx Sin YZt. The charge Q (*) on the capacitor at time t is given by R + q (t^ = VmxSm wt. d t —g-*. from which H(t) = Vm_.sin.0-e + Vm . Sin (Y/t - 0) ' z z with z = (vu1-R* -t- l/ c z ')Vz . The instantaneous voltage & current can he readily found using V (t) « S(t)/c & i (t) = dQ(t)/dt. When the spark gap self fires, the current i(t) = VmSinWt - H flows till t = 7T /w. Fig. 1 shows the variation with time, of driving voltage (Vm Sin Wt), voltage on transmission line (A,~3, C1 if ip I , D^), currents with triggered spark gap (A2, Bg, Cg, I>2) and with un triggered spark gap (A^, ^ ) for 4 valves of charging resistor (A,B,C,D correspond to R = 0.01j 0.5, 1&3-times the impe- dance posed hy the transmission line at driving frequency). Case of triggered spark gap; In triggered case the gap & pressure in spark gap is so adjusted that no self tiring can oecure. A trigger pulse is applied only when the diodes are non conducting i.e . in the second half of the charging cycle. After the trigger is applied the spark gap fires and energy is deposited in the laser plasma. But as the diodes are reverse biased no current can flow from the transformer though the spark gap. In triggered case no charging resistor is required as the current is limited by the impedance posed by the capacitance at the driving frequency. A small resistor or a circuit breaker may be incorporated to take care of accedental self firing. Fig. 2 shows the variation of energy stored (Es) on the trans mission line as a function of charging resistance for the two cases. 2 The energy stored is normalized to maximum storable energy (= 4 CVffl ) and the resistance is normalized to the impedance of line at driving frequency. Fig. 2 also shows ratio K of the energy dissipated aocross - 17 - the charging resistor to the energy stored on the transmission line fdr the untriggered (k (NOTBG)) and triggered (k (TBIGD)) spark gaps. It is clearly seen frcm fig. 2, that, the dissipation in the resistance is considerably reduced if triggered spark gaps are used . This relaxes the power ratings of the charging resistor and the transformer. These results were varified on a transmission line with C = 112 nF, with driving voltage Vm = 20 KV, at 50 Hz. The experi mental points are represented on fig. 2. The points corresponding to the K (50THG) lie above the curve .This is because the curves axe drawn with assumption that spark'gap flies exactly at Vmax, but in practice, after first shot, the spark gap fires at a little lower voltage. Acknowledgement: I am g ra te fu l to Dr. B.p. Bhawalkax, Head, Laser Section, BAKC for his constant interest, encouragement and majy fruitful- discussions. 18 - Vm SIN WT 0-9 LlI o A3 < _i -0-7 § 0-6 o UJ M -3-5 —I < (fc.- 2 cc o z NORMALIZED CURRENT - 0-2 C3 TIME msec. FIG.I THE VARIATION WITH T^€,0F DRIVING VOL AGE, THE VOLTAGE ON THE TRANSMIS SION LINE AND CURRENT FOR TRIGGERED SFARKGAP 8. EXTRA CURRENT FOR UNTRIGGERED SFARKGAP FOR VARIOUS CHARGING RESISTORS. Es \ EXPERIMENTAL POINTS q O Es x K(NOTRG) KtNOIRG \ KITRIGD) Cd 0-L / KITRIGD) \ » u 2 3 2 3 NORMALIZED CHARGING RESISTANCE FIG2.TFE WDATION WITH R IN Es, KITRIGD) 8. K(NOTRG) POLARIZATION PROPERTIES OP INTERNAL MIRROR MULTIMODE He-Ne' LASER D. Sen, P.N. Puntambekar, H.S. Dahiya and V.T. Chitnis National Physical Laboratory, New Delhi Considerable work has been published on the theori- tical and experimental aspects of the polarization proper ties of gas laser sf1-°)Jn case of multimode oscillating gas lasers, it is now well known that successive longitu dinal modes of a He-Ne laser operating at 632.8 nm. show a tendency to be linearly polarized at right angles to each other. In the case of a laser with two modes only, this property has been used for the frequency stabiliza tion of the laserv-®).But the behaviour of a laser with three modes is different from the one predicted above. During our work on the frequency stabilization, we have investigated these properties for a laser (Spectra-Phy- sics) which can oscillate either in two or in three modes depending upon the stabilization conditions. The observations were made by- stabilizing the laser and looking at the mode structure with the help of an Fabry-Perot etalon and a R.F. spectrum "analyser. Under normal conditions, the laser oscillated in two modes when the modes are symmetrically placed with respect to the line centre and the frequency stabilization was achieved by monitoring the intensity of the tv/o modes with the help of two photocells and a heating coil wound on the laser. The coil is heated or allowed to cool so as to maintain the two modes at nearly equal intensity. (i) Two mode case:- The output observed through a Fabry- Perot etalon showed two sets of rings, vanishing alter nately as an analyser is rotated in front of it. The R.F. spectrum analyser showed a beat frequency 0^547 MHz. _ The tv/o outputs are now interchanged in the controlling cir cuit and the laser is again stabilized. The F.P. etalon showed a polarization flip i.e. the direction of polari zation of the two modes also interchanges and the R.F. spectrum analyser showed a shift.in the beat frequency spectrum by^OO KHz. 6K0W& Any isotropic gas. laser oscillating in multimodes A saturation induced polarization preferences and in the case of two mode He-Ne laser, cross saturation effects of the modes and also mode-mode interaction induces a prefe rence for well known orthogonal linear polarization(3). In addition to this if the laser has an anisotropic ele ment in the cavity, the cavity mode of lowest loss will occur for light plane polarized along or perpendicular to to the optic axis of the element. Such a system causes the THIS PAGE WAS INTENTIONALLY LEFT BLANK 21 - easily if we assume the sequence of the cavity resonances in th e t h r e e modes a s o-.tt, t , t» » v3 r z With a frequency diffe rence between and as-300 KHz and that between 77, $nd Although we have mentioned above that the cavity resonance conditions can not be generally met for both the components the above beat spectra can be explained only if we assume the simulteneous oscillations of both the compo nents of all the three modes. Following the sequences as previously i.e. «*, tt, with their simulteneous oscillation, we expect to get three beats at~1092 MHz.viz v , TTa T7} References; 1. W.M. Boyal and M.B. White, Phys. Rev. 147(1966) 359 2. W. Haeringen, Phys. Rev., 158 (1967) 256 3 . D. lenstra and G.C. Herman, Physic a 950 (1978) 406 4 . E.K , H a s e l, O p t. Commun. 31 (1 9 7 9 ) 206 5. Ta. A. Vdovin, M.A. Gubin, V.M, Bnachenko and E.D. Protsenko, SOV. J. Quant. El. 5 (1973) 297. 6. W.M. Doyal and M.B. White, J. Opt. Soc. Amer., 55 (1965) 1221. 7. S.K. Bennet, R.E. Ward and D.C., Wilson, Appl. Opt. 12 (1973) 1406. 8. R. Balhorn, F. LeUowsky and D. U llrich, Appl. Opt. 14 (1975) 2955. Excitation Transfer by Atom-Atom and ELectron- Atora Collisions among Excited States of IIeon V.D. Dandawate Standards Division National Physical Laboratory, New Delhi-110012 The interaction of a strong radiation field with atoms of the active medium of a gas laser in the laser cavity, leads to changes in the population of the atoms in different excited states. These changes are maximum for those states which are directly involved in the laser transition. Erom these states the population changes are transm itted to other close-by energy states by atom-atom and electron-atom collision processes. These changes in the population of excited atoms can be studied by peroidically interrupting the laser radia tion field and measuring the resulting changes in the in tensities of the spectral lines originating from those excited states. These intensity measurements provide a nreat deal of information about atomic properties and ( 1 , 2 ) collision processes of the active medium in the laser c a v i t y . ' . ' To study some of the atomic properties and collision processes of neon in presence of helium, a small cell filled vith helium-neon mixture and excited by DC power cur ply t is placed in the cavity of a He-Ne laser operating a t 6 3 28A*e The excited neon atoms in the cell are thus subjected to the influence of the strong radiation field of the laser, which is periodically interrupted by a - 23 - chopper. fhd changes In the spectral line intensities in the light from the side walls of the cell are detected a ®8 measured, using a monochromator, ph.oto«*ffiultiplier tuc‘ and a lock in am plifier. It is noticed that a large number of energy levels of neon are connected by atom-atom and electron-atom colli sion processes with the upper O s2) and lower ( 2p^) levels of the 6328A° laser transion. Interaction of the fa^ level with some of these states are investigated and these states are sorted out for the two types of processes. Within the small energy difference between the interacting levels, the excitation transfer by atcm-atom collision does not appear to follow any selection rule except for the magnitude of their energy difference, fills excitation transfer process resembles the collision process between colliding atoms at resonance as described by H assa^ ^ and Burhop. On the other hand the excitation transfer by elee* troa-a&om collision, which occurs a® a step-wise excita- tion, obey certain selection rules, 5 It is known that the lowest state of aeon core 2p is an inverted doublet 2p y 2 , 2p^y 2 and the separation of the energy levels of this doublet is the result of the strong ap in-orbit coupling in the cork^. She spin#-or bit Interaction in the core is of greater magnitude them the interaction between the cor® and the running electron, therefore, the excited states for each configuration ~ 24 - split into two groups. The results of the present investigation show that the excitation transfer by electron-atom collision occurs only among those excited states of neon which belong to the same group, i.e. only among those states whose total angular momentum of the core Jc, is same. Moreover, only those excited states in which the spin of the valence electron is conserved .are found to interact. Thus, the selection rule for this process is AJCQJ!;e = 0 , and ^•selectron ~ 0 This electron-atom excitation transfer process in neon from 3b ^ state to 4a1,' , 4s"' and 4s*'(Paschen notation) is further studied on the lines suggested by Parks and Javaii^, and Khaiki^^. The changes in intensities of the spectral lines originating from these energy states are measured at different electron densities. From these measurements, the excitation transfer per particle, the probability of excitation transfer, and the ratio of total probability of electron de-excitation to the total pro bability of radiative de-excitation, are determined for these levels of neon. References:- (1) J.H. Parks and A. Javanj-Physical Review 139, 1351 (1 9 6 5 ). . (2) A.S. Khalkinj-Physics of Atomic Collisions, Proceed ings of-the P.K. Lebdev Physics Institute Vol. 51 p. 93 (1971) Ed. by Aead. D.V. Skobel'tsyn. - 25 - (3) H.S.W. Mas say and E.H.S. Burhop .-.-Electronic and Ionic Impact Phenomena (Oxford U niversity Press London) (-1952). ( 4 ) E.U. Condon and G„H. Shortley^-The Theory of Atomic Spectra; (The Syndicate of the Comhridge University Press) (1963). - 26 - ' T ilS S M S Q LV aD STyBItiS IS. PULSBS ARvKS IQS LAsBR V, S a r i , G> G h a k sa p a n i, P . B.K . S an aa P»Razaazaohana Bao and P,VerikatesuazSLu Lasers and Spectroscopy Laboratory Indian In stitu te of technology, Kanpur**20801 6 * Pdleed laser sources with peak power of a few hundred watts, 5 0 |L sec duration and with a repetition rate of 50pps is a very useful tool for many investigatio ns with lasers. Pulsed ion la-ser-s with necessary optimi sation can meet these rezpiireman t®. She present work is intended (1} to opsimize the one metre ptilaed Argon ion l&ser{2) to fabricate and optimize a 2.5 metre Argon ion laser (3) to understand the excitation mechanism by carry* iftg out the time resolved measurements of the doppler line widths for laser transitions. She laser tube shown in fig. consists of a discharge tu b e o f 4mm internal diametre, 1 meter long terminated on either side by quarts windows set at Brewster angle (.55° 5Q6)« Hie tungsten wire lead-throughs contained in a glass cup filled with Indium/Tin are used^ for cathode and anode, fhe low melting point metals are used to protect the cathode to glass seal from positive ion bombardment. More details about the fabrication are presented in the 2 earlier paper from our group . She Argon pressure is monitored in the present system from 50-1 microns using a pir&ai gauge (GVCj. Both pure and commercial Argon gas have been tried witheut any apparent change in the power output, fae current thorough the plasma tube is measured, using s coaxial type current probe having a response time less thus. IQn secs and resistance of 0.0625 ohm s. The voltage across the plasma tube is measured using a Tektro nix high voltage probe. The laser power output is measur ed with a photodiode {Type 14QA BS R G-) of 1 cm diam eter with a response time of 10 naece. Care is taken t o o p e r ate well below the saturation region of the photodiode by introducing a ground glass plate as the ecatterer. The - 21 distance between the scatter®!* and photodiode is measured accurately to calculate the laser power output. All the three pulses are photographed using a Tektronix 545A oscilloscope with M plug in unit. The D.C. voltage is measured with the help of Hewlett-Packard VTVM. For collection of data a charged capacitor is discharged through the plasma tube ( instead of a transmission line type storage network). Experiments are carried out with plasma tube diamet e r s 2mm, 4mm, 6mm and Smm, all identical in length. For all these tubes both Argon fill pressure and voltage are vari ed and current,voltage and optical pulses are recorded. In each case input power, into the plasma,plasma impedance, energy in the plasma and efficiency of the laser are calc ulated. _THes.e data are important for scaling and optimi zation of the laser. Variation of laser output, plasma impedance with different parameters are.not presented in the abstract due to lack of space. For 4mm diameter tube, using these data the transmission line power supply is optimized. The typical, optical and current pulses are reproduced in the fig. However as shown in the fig. for capacitor discharge two prominent laser pulses are obser ved. For smaller diameter plasma tubes* two prominent laser pulses appear whereas, for higher diameter plasma tubes the first pulse is more prominent at the expense of the second pulse. For Smm plasma tube only first sharp pulse is observed. At high pressure, only a broad peak appears. At low pressures an in itial anti very narrow ( l - 2psecs) pulse is observed along with the broad pulse. As the pressure is reduced the intensity of the in itial peak increases. The first peak is attributed to the single atep^ or cascade^ excitation mechanism produced by fast electrons whereas the second peak due to ffiul tistep^ ex cita tion due to slow electrons. To analyse the excitation - 28 - mechanism an attempt has been made to carry out time res olved doppler line width measurements for laser transit ions. Preliminary data are not very much conclusive in this direction. Using the available data a 2. 6 . m e te r Argon ion laser is fabricated and lasing action Is obser ved. Since proper cavasity mirrors are not available we could not optimize this system in this regard. Furth er work is in progress both to optimize the ' 2 . 5 m e te r laser, and tovref inel Doppler line width measurements. Authors wish to thank Department of Science and Technolo gy for financial assistance §nd to Piar Singh for experimental assistance. . ■ References: 1. Simmons W. W. and Witte R.S., IEEE J. Quant.Blec. QB- 6 , 648 (1970). 2. Rama" S astry K. S, Sivaram A. , Bamamohana Rao,P. Bamachand- ra Bao D,Venkateswarlu P .,and Kumar U.Vi,presented at Laser Symposium IIT,Kanpur (1979). 3. Bennett W.R. J r., Knutson J. V/. J r.,Mercer G.N. and Detch J.L. Appl.Phys.Lett.,4,180,1964. 4. Rudko R.I. and Tang C.L. Appl.Phys.Lett.,9,41(1966). 5. Gordon E.I.,Labuda E.P. and Bridges W.B. Appl.Phys. L ett.,4,178 (1964). SHiso "\AAAr hFI V IK F Ih - 62 - - 30 - INTENSITY DISTRIBUTION IN EXCIMER TRANSITIONS o f NeF, ArF and KrF. V.7. Itagi and B.S. Bhalekar* Department of Phyaica Marathwada U niversity Aurangabad 4 3 1 0 0 4 , I n d i a ♦Mahatma Gandhi Mahavidyalaya, Ahmedpur. Rare-gas monohalid'e excimera have been proved to be efficient high power laser m edial^. These exci- mers are produced in the reactions of electronically excited rare-gas atoms with halogen or halogen contain ing compounds-?-?. An excimer system consists of bound excited state which can radiatively decay to a lower dissocia tive or continuum state and this-transition gives rise to a diffuse or continuum spectrum. The spectral distribution is determined by the !continuum line shape® • ‘ In the present work we report the calculated intensity profile obtained through equation ( 2 ) in th e excimer transitions of NeF, ArF and KrF. - 31 - Earlier workers have used the harmonic oscil lator potential for obtaining iv'> and i f y were obtained either by using flat potential or a combina tion of flat potential and .an infinite wall on one side^. We, in our calculations have used the potentials obtai ned by ab initio calculations and corrected for spin orb it interact ion 10. Both the upper state bound wave • functions and the.lower state continuim wave functions have been computed by using WKB method. The theoreti cally derived profiles have been compared with the experimental profiles 1 1-13 in fig. 1 . The agreement is reasonably good. This kind of theoretical calculations can be used to explore the other possible excimer transitions which may be good laser candidates. One of the authors (BSB) is' grateful to UGC for the award of the Teacher-Fellowship. REFERENCES 1. S-K-Searles and G. A. Hart, Aopl.Phys.Lett. 27, 2 4 3 (1 9 7 5 ). 2. J.J.Fwing and C.A. Brau, AptiL.Phys.Lett.2/, 3 5 0 (1 9 7 5 ). '3. C ..A-Brau and J .J .Ewing, Appl .Phys .L ett .27 ,435( 1975) 4. J.A-Mangano and J.H.Jacob, Appl.Phys.Lett.27,• 4 9 5 (1 9 7 5 ). 5. J.J.Ewing and C. A-B ra u , P h y s .'R ev. A12,129(1975) . 6 . M.F.Golde and B.A. Thrush, Chem.Phys.Lett.29, 4 8 6 (1 9 7 4 ). 7. J.Tellinghuisen, A.K.Hayes, J.M.Hoffman, and • G.C.Tisone, J.Chem.Phys. 65,' 4472(1976) . 8 . F'.H. Mies, Mol.Phys. 26, 1233( 1973). 9. M.F.Golde, J .Mol. Spectrosc. 58, 261(1975).. 10. T.H.Dunning Jr. and P.J.Hay, J.Chem.Phys. 69, 134(1978) . 11. J.K. Rice, A.K.Hays and J.R. Wodworth, Appl-Phys. Lett. 31, 31(1977). 12. T.R.Lorce, K.B.Butterfield and D.L.Borker, APPl. Phys.Lett. 32, 171(1978). 13. C.A.Brau and J.J. Ewing, J.Chem.Phys. 63, .4640(1975). £x j= Co Lc l j LdF e o i KrF « 6 - O 3 - u K) 1 0 5 '92 153 >/Wm. C o /-\p4T2I 6\0U o r £XP£BIJI\LMT^L J^nd C^LCUlJ^T£.o l«TLrt5 l*r^ P C 2 P T 1 1 P S Toe X e F ^ r F L^\ Effect of Tripropylamine in Ballast Besistance Type TEA CO.. Lasers U.K.Chatterjee, U.Nundy, A.K.Nath and E,.S.Shikarkhsne Laser Section Bhabha Atomic Besearcb Centre Bombay-400085 We report the effect of a low ionization potential additive- tripropylamine (TPA,IP=7.23ev) on the electric discharge and laser performance when mixed in small amounts with a normal laser gas mix ture of COg, Eg and He. The different parameters of the ballast resistance TEA COg laser with pin-plane electrodes in which the exper iment was conducted are: Active length = 80 cm, number of pins =130, separation between pin and plane electrodes = 3 cm. Optical cavity length =110 cm., Mirrors= reflecting gold coated concave mirror and output coupling uncoated Ge mirror (36)» reflecting). Energising system — 2 stage Marx Bank, 0=50 nf/stage. Nominal operating voltage = 30 KV, Gas mixture= 1:1:4::C0-:B_:He Output energy = 60 mj for stored energy of 40 j(600 Jl~ A~ ) The output coupling is a mirror of far below optimum reflectivity The condenser value for Marx bank and input energy density is higher than optimum. In order to mix the small amount of vapour TPA with the laser gas,the gas was flown otter the surface of TPA liquid. Our findings are plotted in fig.1. We observe from this figure that with the addition of small amounts of TPA: (i) the output energy increases considerably for the same stored energy in the Marx bank (ii) the operating range of pressure gets extended. (iii) Purthemore the visual inspection of the electric discharge and the laser b u m pattern show that in. presence of TPA the dis charge becomes more uniform and diffuse and constriction of dis-; charge is observed at much higher pressures. (iv) the. lasing volume increases. In another helical ballast resistance TEA COg laser system with same stored energy and 80%> reflecting ZnSe plane output coupling mirror the improvement in laser output energy in presence of control led amount of TPA in 1:1:4 mixture of COg, Hg and He at 400 torr - pressure was observed to be 4C$. The output energy was 2.0 joules . with 0.2 torr of TPA in the laser mixture. - 34 - Several authors have discussed the various mechanisms by which controlled amount of TPA and other low ionization potential additives in u.v. preionized TEA CC>2 laser improve the discharge quality, enhance the operating range of pressure and input energy, the laser gain and the pumping efficiency. These can be summarized as the followings; (a) modification of electron energy distribution due to ionization of additive molecules either by direct electron impact or by Penning 3 type processes with metastable Eg levels . This will reduce the average electron energy which would lead to better pumping 6 efficiency . (b) enhancement in the background electron density1 by u.v. preioni zation which make the discharge more uniform and stable against ■ a rc in g . We disooas our results in terms of above roles of TPA in the laser mixture. In our ./present experimental condition the improve ment in the output energy aid extention of operating pressure range each almost by 2 times cannot be attributed to any single mechanism operative in presence of TPA. At low pressures where uniform dis charge is assured, the improvement in output energy is mainly due to the modification of the electron energy distribution. To substan tiate this we measured the discharge current and voltage across the discharge in gas mixtures with and without TPA. We found that in presence of small amount of TPA the discharge current was significan tly more and voltage across the discharge was less. In a normal laser gas discharge the E/p across the discharge is such that the average electron energy at this E/p is beyond the optimum value at . which the excitation of I?2 and C02 molecules is maximum. At lower E/p the average electron energy is closer to the - optimum'and therefore the pump efficiency increases. Further more, visual inspection of the discharge shows that in presence of TPA, the glow is diffused and faint and there is only a bright point at the tip of pin electrode. This shows that a comparatively lower ! fraction of electron energy is spent for electron excitation of the gas which originates the radiation. 1 In normal laser gas mixture at high pressure, on the other hand, each individual discharge co n stricts - 35 - deteriorating the discharge quality and heating up the gas. Both are detrimental for laser output energy. In presence of TPA the discharge becomes more diffused and uniform end constriction occurs at relatively high pressure. The diffused discharge in the presence of TPA is attributed to the higher volumetric preioniaation electron density generated by radiation of the initial carena discharge at the tip of each pin electrode. Thus we observe that ib ballast resis tance TSA laser the small amount of TPA in the laser gas mixture modifies the electron energy distribution and makes the discharge more uniform and stable. We would finally like to say that we are continuing a more detailed investigation of discharge current and voltage in different gas mixtures with and without TPA at different input energy densities to’ find out how the attachment and recombination coefficients of the laser mixture are modified in the presence of controlled amount of TPA. These coefficients affect the stability of discharge n considerably . These details will be reported in a future publica tion. References; 1. Seguin, E.J.J., Tulip, J., and Meker.D, Appl .Phys .Lett. 21, 527 (1973) 2. Levine, J.S., and Javan, A,, Appl. Phys. Lett., 2±, 238 ( 1974 ) 3. Reitz, B.J. and Olbertz Appl.Phye.Lett. 26, 335 (1975) 4. Morlkawa,B., J.Appl.Phys. 48. 1229 (1977) 5. Hubner,H. and Homann Ch., Appl.Phys. 1.2. 211 (1977) 6. Higban, W.L. and Bennett, J.H., Appl.Phys.lett., J4, 240 (1969) 7. Nighan, W.L. and Wlegand, W.J., Phys. Bev., A JO, 922 (1974) ENERGY mJ & o 0 200 30 0 600 700 0 7 0 0 6 500 0 0 4 300 0 0 2 100 RSUE TORR PRESSURE ™ ™ 36 = - 37 - Multiline Injection Locking In TEA CO- La3er A .X .Hath and U .K.Chatter jee Laser Section Bhabha Atomic Research Centre Bombay-400085 . The injection locking technique of producing multiline/double band gO^ laser oscillation, basically consists of a CW or pulsed laser signal comprising of several rotational-vibrational transitions each oscillating on single longitudinal mode produced in one or more low pressure master oscillators which is injected into the optical cavity of a slave TEA CO,, laser^. When the frequency of each injected rotational line'is very close to the cavity mode frequency 2 of the slave oscillator, its output oscillation is locked to the injected signal. The output then consists of simultaneous multiline oscillation each oscillating on a single longitudinal mode. Using this technique several authors have reported multiline oscilla tion. Sheffield et al^ reported simultaneous laser oscillation on p( 16), P( 18) and p(2Cl) lines of 10.6 jm band with uniform intensity distribution and found a threshold value in the range of 1 W on each 4 line of the injected signal for complete locking. Cellert et el reported a threshold power of the order of few nfif for either P(2$) -H(20) pair or p(22)-R(26) pair of 10.6 jim band. These observations show that when the P(20) line is present in the multiline spectrum the threshold power for complete locking of the slave oscillator is more. We have also produced multiline oscillation in a hybrid TEA CO- 5 laser using the same principle . Here the low pressure oscillator acts as the master oscillator. In our experiment we observed that the temporal profile of the multiline laser pulse changed with different 5 injection signal power level . The purpose of this presentation is to explain $he different threshold power of the injected signal for locking the slave TEA oscillator oh to different spectrum and to predict theoretically the evolution of a multiline laser pulse for different injection signal power levels. We have used the theoretical 6 „ model developed by us for multiline oscillation in TEA CO,, laser. In the model the injected signal in, the TEA laser is taken as the - 38 - initial photon density in the computation. Other parameters in the calculations are the following: Saa pressure = 400torr, Mixture: C0„:B„:9e = 1:1:4; Gas temperature =400 K, Discharge 13 electron density = 10 /c.c. The output intensity distribution on ?(16), P(18), P(20) and P(22) of the 10.6 ji»band for different injec tion signal power densities is shown in fig.1-a. The pulse shapes of the total intensity have been shown in flg.l-b. for different in- , jection signal power levels.' For each power level the intensity dis tribution on all lines of the injected signal has been assumed to be uniform. From these figures we observe that the spectral distribution of the TEA laser output improves with the power of the injected sig nal and the onset of laser action is earlier. The peak intensity reduces, however its F>VHK increases such that the laser energy remains almost the same. In the inset of^?-b the typical experimental laser pulses of a hybrid TEA 0 0 ^ laser have also been shown for comparison. Better spectral distribution could be obtained if instead of equal intensity on different lines of the injected signal, we took different intensity distribution. In table-1 results for different intensity distribution of the injection signal. In the last array we can see that in a typical TEA CO^ laser (small signal gain ~ 0 * 5% cm-1) almost equal' intensity on P(16), P(18), P(20) and P(22). can be obtained for * 2 - injection signal intensities of % 3,1 and 4 W/cm on th e above l i n e s respectively. The intensity distributions on P(16), P(l®) and P(22) lines without the P(20) line have been shown in fig .2. Comparison of fig.1-a andvflg.2 shows that in the absence of P(20) line, quite uniform spectral distribution can be obtained at relatively low injec tion signal power level. This is because of the anomalously higher gain of the P(20) line?, compared to other neighbouring lines and in order to generate uniform intensity distribution with P(20) line in the spectrum a higher power injection signal is required. The earlier onset of laser action with higher injection signal la due to fact that the laser oscillation starts from higher initial photon density. More over the presence of hi$i level radiation prevents the growth of large population inversion, precluding the formation of gain-switched peak. This causes reduction in the peak power at higher Injection signal. Since the same energy is stored with ulea in the TEA laser - 39 - 2 Z, 2. c 0/ 1 o of Vo' V) 0 3 -1 10 InjeciioA S ig n al ?oere-r Jjex\6ii^ ' S pectral diiixVlouiion oj. rnuJdiline. i.wiettvoA locked. T EL A CO2. Lasfe'f ^tfx dUjjcxtrvk ^oi-rt'x-S cj injec.ii.6tv Si-j-mL Jjteditiei by tki model. 5 4 r2 -3 a —•< H*- 500 nit*, s- 1 « t -* 0 o-4 v a . t C /Aiee.) ^ levrvUmo-L loxol^ie Uvu* *puU>e ^-tr$ turn du^txert . T tnjecicrk si .0 tvxl Level dieted V>j tv* trxedel InS^i E._ >x.bexXxnexi,a.L iemLyroi ^xc^le TABLE -1 Intensity distribution Intensity distribution of the of injected signal TEA laser output (W/cm2) (iff.Y/cm^) P-16 IS 2 0 22 P-16 18 20 22 1 2 1*5 1 2 0.5 0.7 ’ 1.2 •0.6 2 5 3 1 4 : 0.8 0.9 0.82 0.8 -3V S i H 1 -s', ti 1 o r - ' ■r> d _J ■ ii iC° io L2 io VA f tirr J-Yuexts-d. Sin T voi Ecv-rCT T jervSli^ ne. i\ni Heferences: 1. Izatt, J .R. ,Budhiraja,C. J. & Mathieu,P.,IEEE J .Quant .Elect.Q5-13, i 396(197?) ••2. Lachambre J.L. et al, IEEE J .Quant .Elect .££=12,756(1976) ■;3» Sheffield,R.L.,Kazemi,S and Javan.A.,Appl.Phys.Lett.29.588(1976) ;4. GeliertjB, Handke,J. and Kronast B.,Appl.Phys. 19.257 (1979) 5. I'.ath A.K. and Chatterjee U.K. -Presented in Symposium on Infrared : Technology and Instrumentation, B.A.3.5., fcarch 5-7 (1980). ■ 6. Nath,A.K., Chatterjee,U.K. and Bhawalkar D.D. ,Opt. Quant .Electron. 12, 245 (1980) 7. Singer,S.,IEEE J. Quant. Elect. QE-10. 829 (1974) -'4 1 - Discrepancy Of Q„ Concentration In The Dissociation Prodhgxs Of A Sealed TEA C0„ laser *D.J.Biswas and U.K .Chatterjee *M.D.R.S; Physics Sroup, Bhabha Atomic Research Centre,Bombay-85- Laser Section, Bhabha Atomic Research Centre, Bombay-85 Considerable disagreement exists among various investigators concerning the discharge gas products in a COg laser [1-6] . However; the accumulation of CO and Og in appreciable amounts has been confir med by all. These are produced by the dissociation of COg in the glow discharge according to the reversible reaction 2 COg 2 CO + 02 But their observations regarding the presence of other species differ. NO and N02 was observed in considerable amounts at pressures of 5 torr in D-C discharge by Tannen et al [2]. Scholzau et al[3] have observed NO, NOg and 0^ in ppm level in a flowing gas TEA COg laser. A number of researchers could not detect the presence of any of the oxides of nitrogen up to a fairly good detection sensitivity [4-6] . Mass spectrometric measurements failed to give the true concentration of CO in the presence of large quantities of Ng both of which have, the same molecular weights. This ambiguity has been removed in the present investigation by use of gas-chromatography, where the CO and Kg peaks are veil separated (Fig. 1). The, laser used for this investigation was a Beaulieu [9] type helical TEA COg laser of 64 cms active length with a gas mixture of COgiKgtHe:: 1:1:4 at pressures of -.350 torr. A 0.02 uf capacitor.', charged to 30 KV was switched across the electrodes at 50 ppm. A sample collector of 5 ml volume, after its evacuation, was used for collecting the degraded laser gas at specific intervals. The small volume of the sample collector did not change the quantity of gas in the laser tube by any appreciable amount. Each sample was then analysed gas-chromatographically. The results of the analysis are shown in figs.2-3* From fig.2, it readily follows that ail any instant the concen tration of Og is less than half that of CO and this difference viz [CO}- 2[Og0 increases with number of input pulses. To our knowledge, - 42 ~ HELIUM oz CARBON OXYGEN l/lQ. MONOXIDE TIME IN MINUTES START FIS.1: A typical gas-chromatogram of the degraded laser gas. Peaks due to nitrogen and carbon monoxide are well separated. co, concentration concentration 2 TIMES Oj CONCENTRATION n , concentration cr _ LU OCCD LU < 1000 2000 3000-1000 SOOO 60ffl 7000 2000 1000 6000 8CD0 TOTAL NO. OF PULSES total n u m b e r of p u l s e s 0 1000 2000 0000 i.000 Sofxl Bofao 7000 TOTAL NUMBER OF PULSES fig.3: Variation of laser s- Pig.2:Concentrations of carbon mono- energy with total num I xide, oxygen and carbondloxide versus ber of input pulses. total cumber of pulses. Inset shows the independence of nitrogen concen tration with input pulses. * The error in the gas-chromatographic measurement of the concentrations were too small to be shown on this scale. - 43 - this is for the first time that this discrepancy is clearly demon strated . CO cannot be produced from any process other than the dissociation of COg and this is confirmed by the fact that the amount by which COg is depleted at any instant is equal to the amount of CO generated (Pig.2). Since the estimated oxidation of the copper pinsy which form the electrodes, was calculated and found to be negligible >- the absence, of Og in such large quantities indicates another parallel reaction ( or reactions) occuring inside the reaction chamber. More over, the concentrations of COg, CO and Og as well as the output pulse enfrgy continue, to remain unchanged, after 5000 input pulses (figs.2' & 3). The departure of the observed Og concentration at equilibrium from, that needed to satisfy the condition "(CO)/2(0g)=2/T1 is almost 26$. There was no detectable amount of any oxides of Nitrogen, a fact corroborated by tiie constancy of Kg concentration during the experiment ( in"set of fig.2). The only possibility appears to be the formation of ozone (5-8,10) by the following reac tion process [5). 0 + 0g+M;==s 03 +M ° + °a Og + Og •M1 may be Og, COg, Ng, .He or 0^ (11). Thompson et al (12) observed ‘ that the decomposition of COg on irradiation with uv ■ light also yields C0j3 of nearly 40$ of (Og") at equilibrium through the following process, 2C0g ^ 2C0 + $2 It 02 ^ 2/3 03 CONCLUSION: In a sealed TEA COg laser it is clearly demonstrated that there is a large discrepancy, between the available and expected amou nts of Og produced by the dissociation of COg. Prom the foregoing discussion, the possibility of ozone formation to account for this reduced quantity of Og appears to be quite likely. HEPEHBKCES; 1. J.A. Macken et al, Bull. Am.Phys.Soc, Vol-12,p.669, 1967. 2. P.D.Tannen et al, IEEE J.Q.E.L. Vol.QE-10, Ko.1, pp.6-11, 1974 3. H.S.Scholzan^et al, Phys.Lett, Vol.48A,No.3, pp.205-206, 1974. - 44 - 4. A.L.S.Smith, J.Phys.p.,Appl.Phya.,Vol-41, pp.1367-91, 1975. 5«- H.Earube and B.Yamenaka, J.Appl. Phys. Yol.41,' pp.2031-2042, 1970 6. D.S.Static et-al, IEEE J.Q.El.,Vol.11, Ho.9, pp.774-778, 1975 7. A.L.S.Smith and J.M.Austin, J.Phys.B, Pol.7, Ho.6, pp.1191- 1194, 1974. 8. H.G.Buser and J.J.Sullivan, J.Appl.Phya.,Vol.41, Ho.2, pp.472- 479, 1970. 9. A.J.Beaulieu, Appl.Phya.lett.,Yol.15, PP-504-505, 1970. 10. C.A.Jacobson, Ed.,Encyclopaediaof Chem Equna, Vol.5, Reinhold Publishing Corporation, Hew York, 1953* 11. S.W.Benson and A J .Axworthey, J.Chem. Phys. Vol.26, pp.1718-26, 1957. .12. B.A.Thompson et al, J.Phya.Chem. Vol.69, Ho.11, pp.396.4-67,1965. - 45 - Problems of Simultaneous triggering of two coaxial plasma tubes In a C?; CO. Laser. B.L.Gupta, 3.3 .iiarayan & L.K.Kukreja - Laser Section,3 .A.5.C. ,Trombay, Bombay-400 085. Abstract Simultaneous striking of cotpled tubes poses a big problem in coaxial CVZ-COg laser. This paper discusses a circuit using SGRs to strike:, the coupled tubes simultaneously. Introduction In coaxial CV/ CO^ Laser in the range of (100 W to 500V/) output power the length of the tube is divided into sections (Tyte 1970) which are electrically in parallel and optically in series. There are various advantages of the arrangement such as "Seduction in striking voltage (ii)Isolation of optical mounts etc from high voltages for a safe operation and (iii) mechanically simplifying the system. But serious problems arise in such a con figuration because of the slight difference in the striking voltages and discharge characteristics of the. plasma tubes and thus the probability of gettirg discharge simultaneously in all the coupled tubes becomes very low. • In a practical system the various limbs of the plasma tubes are connected through appropriate ballast resistors to a common voltage power supply. Electrically these limbs appear in pairs as shown in fig.(l). After switching on the power, the energy capaci tor starts charging slowly. During this slow charging the limb with lower striking voltage strikes before the other limb of the pair can strike. This results in a large voltage difference between two anodes and a discharge takes place between the two anodes. This prevents discharge in the second, limb of the pair from striking. Initially to overcome this problem the capacitor was charged to a voltage higher than the striking voltage of both the limbs and then the charged capacitor was connected to the plasma tubes mechanically. This improved the striking reliability very much but it was a tedious procedure and not completely safe. Alternately it was thought that if suddenly a step voltage is applied to the two anodes at a time when the slowly rising volt age at the anodes is slightly less than the striking voltage of either of the limbs, simultaneous discharge in the two limbs can be obtained. The resultant voltage should be greater than that required by the limb with higher striking voltage, so that both the tubes strike simultaneously. When the discharge in both the - 46 - limbs strike, both the anodes are maintained at nearly the same potential and there is no chance of.discharge between the two anodes and thus the gap between the two anodes can be made as small as 5 cm. Here, we describe a simple circuit which applies such a step voltage. 2. The Circuit; The circuit is shown in fig.2 and is shown connected to a pair of sections. It applies a step voltage on the anodes from a level lower than the striking voltage of either tube to a level higher" than the highest striking voltage. A chain of SCR's connected in series, operating in the self breakdown mode, is used to realise . required step voltage on the anodes." R^ and are the ballast resistors for the two tubes. Resistor chain B- comprising of resistors r.,, r2 etc. equalises the voltage drop across SCR's and also in combination with B_ applies a d.c." voltage on the anodes which is a fraction (R^/R^+R^J of the input voltage above which the jump in voltage is to be given. The fraction (H^/ai+Bg) .of the input voltage appears across the SCB's. As the input voltage is slowly increased and when this fraction exceeds the self breakdown voltage of the SCB chain, the SCB's breakdown offering a short circuit and applying the full input voltage to the. anodes to strike both the tubgs simultaneous ly. The current is limited by the circuit impedance. The SCB's used here are selected, on the following considerations (i) They should have low leakage current before breakdown so that R^ and Rg.can be chosen to be large, with the result that power dissipated m B^ will be small when the laser is operati*ng(ii) They should have low holding current- so that they remain in conduction state after breakdown for the minimum desired operating current in the tubes, (iii) All SCR's in the chain should have nearly equal breakdown voltages so that "the shunt resistors are also equal. 3. Bxcerlmental Besults: A CVf-COg laser, 3 it long consisting of two sections of plasma tubes of 1.3 m length and anode separation of 60 mm, operating at a total pressure of 21 torr (COgfHgtHe::1:3*S*5) was found to have . striking voltages of about 16 KV & 16.3 KV respectively for the two sections. It was observed that streamers occur much below the strik ing voltage at about 13 KV and cause breakdown of the.SCR's. This and the consideration of the loading of the power supply after the tubes strike decided the voltage step to be around 6 KV(i.e. from i?.5 KV to 18.5 KV). lline SCB's of RCA Type S206QD were used in the circuit and with such a circuit the striking probability was found to be 10G;i. hcijS re nc a : Type D.C. 1970 Carbon dioxide laser in Advances in Quantum Electronics ed. D.W.Goodwin (Hew York Academic) - 47 - S O H i seam FIG .f HIGH VOLTAGE. CONNECTION TO DIFFERENT SECTIONS Rt _A-1 n n n. j wWV » ■ W*A-. ■------AAZ—n I • k , I h. — 1 »br 1 9 ) LASER BEAM SCR S10H0 I RCA) FIG 2. CIRCUIT USING SCR's 48 ■= Some New Laser Dyes M.R. Padhye. T.S. Varadarajan and A.V. Deshpande Department of Chemical Technology U n iv e r s ity o f Bombay, Bombay *+00019 Introduction:- 'An essential constituent of any laser, is the amnlifying medium and in case of dye lasers it is a solution of an organic dye. Since the beginning, the development of the dye laser has been closely tied in with the discovery of the new laser dyes. It was repo rted in 1969 that a survey of approximately one thousand commercial dyes showed that only four of these were useful as laser dyesl. This specificity of dyes for laser action shows that the laser dyes must satisfy certain molecular properties specially optical properties. These, as is well known, are structure dependent and thus laser action becomes a property very much dependent on molecular structure. Principally there are a few classes of dyes which are by now established as commercial laser dyes namely couma- rins, oxazoles, cyanines and quinolines. All these are well know for their fluorescence emission which is the just necessary condition for the dye to be a potential laser dye. The above types of dyes only signify the basic structure from which innumerable molecules are produced by variously substituting these basic molecular frameworks. Some of these derivatives are fluorescent and some are not. Amongst those that are fluorescent, it is the region of fluorescence which essentially determines the tunability range. The other variable of importance in the structural aspect of laser dyes is the quantitative aspect of light amplification or power output for a given concentration in other words the efficiency. Thus the correlation of molecular structure with laser activity of dyes essentially means correlation in the variation of tunability range and efficiency. According to the basic theory the'physical properties which principally form the basis of laser action in such systems consisting of large molecules of complex structure are the life-tim es in the excited emitting state and the quantum efficiency. These in turn are decided by the radiative and radiationless transition probability of the excited state. Thus the measurable experimental parameters on the basis of which the laser parameters can be hoped to be synthesised are the life time and quantum efficiency of emission. In the present paper the results of a part of the extensive programme of work so far completed are being reported. Twelve new substituted-coumarin derivatives are synthe sised and their various properties are studied. Effect of - 49 solvent, the usual optical absorotion and fluorescence• emission spectra are studied to have necessarv idea rega rding the appropriate region for pumping radiation and-' tunabilitv range in case the dve proves useful as ampli fication medium. Among, the other important optical pro$>§= rties their auantum efficiencies are determined. As fok! the life times most approximate idea of highest lim it is., obtained from the integrated intensity of absorption, but' this is no way is a satisfactory method and more accurate' methods of life time measurements are being tried. Out of these twelve new substituted coumarin four are found to be useful for laser action. The dependence of the lasing action on the nature and position of sub stituted group is discussed. Experimental set u p ; - The new substituted coumarins are synthesised and purified. Their purity is tested by IR spectra and also by chromotography. The absorption spectra of these coumarins are recorded on Beckmann DK2 spectrophotometer and fluorescence emission is recorded on Aminco’s spectrophotofluorometer. For the measurement of the Q.E. of these dyes experimental set up fabricated by us is used. This set up is an shown in the fig. It contains monochromator (Jobin YVON - H20 model) and the standard photomultiplier EMI NO. 9781 together with Spex photon counting system. 1’he photomultiolier is cooled by using the electro cooled photomultiplier housing. A point source Xenon arc (60 W) is used as a source of exci ting radiation. A freshly prepared MgO film is used as an ideal scatterer. R esults:- Absorption and emission spectra are recorded by using the different solvents viz. Methanol, DMF and Chloro form. It was found that the emission intensity as well as the emission wavelength in case of some dyes is solvent dependent. This naturally is expected to be reflected in laser emission. Table I shows the list of.dyes synthesised along with their,emission character. The first seven days in the table which are not fluorescent are not considered for the present study and the remaining five were subjected to- more detailed scrutiny. In table 2 are listed the five of twelve dyes that are found to be fluorescent and of these four definitely show th e‘satisfactory laser activity; Their pumping wavelengths emission maxima, tunabilitv range and ouantum yields are shown in table 2 , The Q.E. of these dyes were found from the relation - 50 - Q.E. = no. of photons emitted no. of photons absorbed The set up used to measure the Q.E. is shown in fig. The incident intensity from the xenon lamp is measured by using a perfect diffuser namely MgO. The Q.E. is found by front surface reflection method2,3j4-. While calculating the Q.E. the observations are corrected for the grating sensitivity and photomultiplier *s sensitivity. It is reported that the errors in absolute Q.E. are minimised by using front surface irradiation method. Necessary correction for the refractive index is also made. It is hence believed that the values of Q.E. are quite precise. The laser action of these, dyes was tested by using a Nitrogen laser radiation as pumping radiation and the usual transverse cavity with dye in one cm cell at the spectro scopy division of BARG. All the dyes were tested but those showing fluorescence"emission, were tested more care fully. For these dyes both the solvents and concentration were adjusted till maximum output power was obtained. The total output power and its wavelength wise distribution is yet to be determined. Discussion;- At the outset it must be pointed out that the present series of dyes are entirely different structurally than the commercial laser dyes of the coumarin series. Table 3 shox^s the structure and the wavelength of laser action of some of the prominent commercial dyes. When these structures are compared with the structure of dyes of the present series, it is observed that there is a more complex substituent at position 3 in the present series. Since the object was to develop dyes with wider tunability range extending to longer wavelengths, basically it is understood that range of ir electron delocalisation ought to increase by more extensive conjugation at position 3 next to 3;*+ double bond in pyran ring. It is obvious that any other position of substitution in the phenyl part is not expected to be as effective since it would lead to cross conjugation. The other position is 2 where conjuga tion is only effective provided the double bond is next to ring as in imine structure. To compliment to effect of increased conjugation a suitable electron active group in position ? is expected to be helpful. These predictions were kept in mind while formulating the programme as far as present series of compounds. We are working on some other series also which w ill be repo rted later„ These considerations though theoretically correct have practical lim itations. All conjugative sub stituents do hot yield reouired results because of one of more of the following. The substituent may be - 51 - sterically disposed out of plane with coumarin ring in:, which case conjugation is not possible, the substituent may have chelating groups which may bring about inter or. even intramolecular hvdrogen bonding in which case again’ the. shift may not be as expected. It ls also likely that? fluorescence,characteristically different and since such;. ' and sim ilar structural changes may lead to energy dissi-"* pation and make compounds non fluorescent. On the basis of these considerations it is easy to understand why in the commercial laser coumarins mostly 4 substitution which is the position which can be readily substituted was lined and also to understand why such a large number of dyes synthesized here have been found to be non fluorescent. Among those that are fluorescent and give laser action it is auite clear that the expected extension of tunability range has been achieved. The solvent effect of these dyes is nuite marked because on the substituents there are often centres susceptible to solvent action e.g. hydrogen bonding groups or ring nitrogen with lone pair electrones. Much more work is yet to be completed e.g. to find correlation with yield and life times as also more effe» ctive substituents. It also remains to be seen whet is the relative effect on the power output . of these lasers. ftBFBRENCES t.D.W.Gregg and S .J. Thomas, IEEEJQ B - 5 ,3 0 2 (1969) 2.W.H. Melbuista, J. Phys. Chem. 65, 229 (1961) 3.J.N . Pemas and G.A. Grosby, J. Phys. Chem. 75, 991 (1971) 4.W.H. Melhuish, N.J. Sci and Tech. 37B, 142 (1955). - 52 - Table No. I NON FLUORESCENT DYES Dye No. Chemical Name 1 2 1mlno, 3 benzimidazolyl, 7 diethyl amino co u m arin 2 7 diethyl amino, 3Imido, 2N aldehyde imino co u m arin 3 7 diethyl amino, 3 benzimidazolyl 2N phenyl auanidino imino coumarin 4 7 diethyl amino, 3 imido, 2 imino coumarin 5 7 diethyl amino, 3 benzimidazolyl, 2N phenyl imino coumarin 6 7 diethyl amino, 3 imido, 2N cynamido, imino coumarin 7 7 diethyl amino, 3 carboxylic acid, co u m arin ___ FLUORESCENT DY ES 8 7 diethyl amino, 3,3(1 phenyl, 4 aldehydo) pyrazolo coumarin 9 7 diethyl amino, 3,3 (1 phenyl, 4 cyno) pyrazolo coumarin 10 7 diethyl amino, 2 imino, 3 anilyde co u m arin 11 7 diethyl amino, 3 indol coumarin 12 7 diethyl amino, 3 benzimidazolyl co u m arin - 53 - Table No. 2 Dye No. Wavelength Wavelength Lasingn excitation emission action QE (ran) (nm) in DMF 8 393 487 YSS 0 .9 0 5 9 398 491 YSS 0 .7 2 10 390 476 YSS • 11 435 494 NO 12 440 526 YES o . 90 - 54 - Table III Reported coumarin dyes Dye S o lv e n t X la s (r . XabsW Coumarin 120 HFIP 326 TFE 338 H20 343 MeOH 351 440 Coum arin 2 CH2 CI2 355 . NMP 362 MeOH . 364 , 450 HFIP 378 • 460 C oum arin 1 MeOH 373 460 TFE 388 470 HFIP 398 480 Coumarin 102 NMP 383 470 CS2 CI2 .3 8 6 470 MeOH 390 480 TFE 405 500 HFIP 418 510 Coumarin 30 MeOH 405 • 510 Coum arin. 6 MeOH 455 540 BLOCK - DIAGRAM 5= Source L= Lens F= Filter box M~ Mirror C = Sample holder MC= Monochromator PM= Photomultiplier PC = Photon counter - 56 - OPTICAL GAIN JS THE RANIPAL-S LASER PTE. V.V. Itagi sad B.H. Pawar* Department of Physics Maratbwada U niversity Aurangabad ^31004, India ♦Physical Research laboratory, Navrangpura, Ahmedabad 380009, India. We have earlier reported on the laslag in the commercial whiteners Ranipal and Ranipal-S* Since Ranipal-S is an efficient laser dye in the blue region of the spectrum we have carried out measurements on the gain as a function of wavelength in the lasing spectral range. Huth^ measured the gain of a flash lamp pumped dye laser by using oscillat or-aaplifier combination. It is difficult to apply this method to dye lasers pumped by a pulsed Nitrogen laser, since in this con figuration the gain is very high and it is difficult to distinguish the effect of the probe beam from that of Amplified spontaneous emission (aSB) • SUfvast and neech3 had developed a method to measure the gain in euperradiant (or ASE) high gain poised metal vapour lasers and the same method was adopted by shanX et al*+ to measure the gain in dyes pumped by Nitrogen laser. The gain was dedueed from the measured ASE intensities Iv and I Vo for the respective gain len g th s 1* an d L/ 2 of the aye from the expression we have adopted the same method in the present study. The experimental setup is show in the figure. A dye cell with canted windows was pumped by a nitrogen laser beam of rectangular cross- section focugeed to a line by a quartz cylindrical l e n s . I blackened metal plate was mounted on the movable platform of a travelling microscope and the motion of the platform was restricted in such a way that in one extreme position it allowed irradiation of half of the cell length and in the other extreme position full length of the cell could be irradiated. The intensity of the 1SB for L and V2 was measured at different wavelengths using a Carl Zeiss SH'-2 monochromator and EG and G -460 laser power meter, the intensity gain coefficient av calculated at various wavelengths from the above expression are shown in f i g u r e . The method is simple and is useful to apply whenever the ASE is reasonably strong. The authors are grateful to ISRO for the financial support.- KEFE REICES 1. V.V. Itagi, B.E.Pawar and Sharada Itagi, Inti.Jr. Phvs. (in press). 2. B.G. Huth, Appl.Phys.Lett. 16, 18? (19/0). 3. W-T. Silfvast and J .s . DeecE7 Appl.Phys.Lett. 1 1 , 9/(136/) . 4 . C"T?. S h an k , &. Dienes and w.T .Silfvast, Appl. Phys.Lett. 1/, 30/ (19/0). - 58 - Nitr.ogerv. Laser* Metal si>i|> to block halj the 3om\ Lervcflh. Eh- HfrEglMEUTAL SETUP 18 16 |4 12 10 S 427 429 45! 433 435 437 Wavelength (nm) GAIN OF THE Dy£ 1AS£C AS 4* TuHCTlOU OF WAVELENGTH1. - 59 - EFFECT c r HTDRCGEfl Si: UA.WC D'Yc C,A5£B . QiABACIEA I5 1 i.C S V. KA5ILAMA&I and 8.M. S1VAHAM Department of Physics Guindy engineering College Perarignar Anna University of Technology Madras - 600 025, India ABSTRACT The dye laser characteristics depend largely upon the nature of the dye molecule and upon the environment in which it is placed. The present study is an attempt to understand the influence of hydrogen bending (H-bonding) on the characteristics of 7 Diethyl Amino 4 Methyl Coumarin (DAMC) dye laser. The small signal gains of- DAHC in a variety of solvent environments were determined-and the results were analysed in terms of - solvent-solute inter a c tio n . The small signal gain gives information about the potential and capability of*medium for laser action. It can be experimentally measured using the technique of Shank et al (l). The dye s o lu tio n was tra n s v e rs e ly excited by the Nj laser built by us and from -the’ super- radiant intensities for two different excitation lengths of the dye solution the small gain can be obtained. The' sol vents chosen for this study were benzene representing non polar, non-H-bonding solvent; Q-dichlorobenzene represent ing polar non-H-bonding solvents; and alcohols which are polar, H' bonding solvents. ^ < - 60 - Table 1 show# the relevant solvent parameters and the gain values* Over the range of concentration (2 x 10~3 M to 8 x tO~3H) end N2 laser power (1 to 8 kb) studied the gain showed a linear dependence on both the concentration end pump powers used* Hence the data era presented in terse of gain per eelecule par kw puop power* The linear depen dence of gain with’’ kg laser power indicates that the gain ia unsaturated end hence the slopes of gain Vs Nj laser power plots may be taken as a M essrs of efficiency of these solutions* The gain band width gives the tunable range of dye laser from these solutione* Tablegjindieatee that gain end gain bandwidth are largest for polar- H bonding solvents* leas for polar sol vents and least for non-polar solvents* Since alcohols are Pol mi and N bending in nature, the dye DAMC undergoes both dipole-dipole and H bonding interactions aiaultaneouely in alcohol solutions* But the relative magnitudes of these interactions are difficult to aeaeaa. Nevertheless* H- bonding enhances gain in alcohol solutions and this can be inferred.from Table 1* Except methanol and ethanol the polarity of thexeet of the alcohols studied are similar* But the gain in isopropanol is more than twice as large ee in n-amyl alcohol. Again methanol is mere polar then isopropanol but the gain is lower in methanol than in isopropanol* Table 1 shows that the peek gain occurs around 4S5 nm for all those alcohols and the gain bandwidth is eppxoxi- - 61 - eately the same for ell these solvents and is about 60 nm. But the magnitudes of the gain varies greatly among the various alcohols (see Table 1) and can be explained on the basis of the difference in the H•bonding capacity end polarity of the solvent and the vibrational deactivation loss in the M-toonded solute-solvent combination. Our study ehcws that for e polar dye such as OAMC, poler H-bonding solvents are the most evitable environments for producing high gain and broad gein bandwidth; Among the typical H-bonding solvents such as alcohols studied in this paper isoalcohole are better than normal alcohols end ethanol and isopropanol; are the beet solvents amongst the alcohols studied. The. authors wish to acknowledge,the financial assis tance of Department of Science and Technology, Govarnmant of India to carry-out this work. REFERENCE I. C.V. Stank, A. flienes and W.T. Silfvgst, Appl. Phys. Lett. 17 (1970) 307. Table 1 % Solvent effects on the characteristics of DAMC dye laser Wave- Gain Slope of Dipole - C337 length of Band Solvent gai'-i D ielectric peak gain width moment (10+4 litre d O " 16 or o'" ( Debyes) cons tant mole ^. cm ^ ) ( nm) ( nm) KW“1 .mole"1) A. Benzene 0 .0 0 2.20 1.3 415 20 0.24 . B. O-dichlorobenzene 2.26 9.93 0.9 430 40 0.42 C. Methanol 2.87 32.63 1.3 458. 60 0.56 1 01 Ethanol 1.69 24.55 1.1 4 57 60 0.90 N) n Butanol 1.66 17.70 1 .0 452 60 ■ 0.60 1 n Amyl alcohol 1.79 17.70 0.9 .4 52 60 0.44 ' Isopropanol 1.66 19.92 1.1 452 60 ' 1.00 Isobutanol - 1 6.56 0.9 452 60 0.70 Isoamyl alcohol 1.66 10.90 0.9 453 60 0.61 2-Ethyl hexanol - - 0.9 . 451 60 0.60 A - Non-polar non-H-bonding solvent ^337 *s molar extinction coefficient at Nj B - Polar non-H-bonding solvent laser wavelength C - Polar, H-bonding solvents - 63 - B ggicisrcr OF SiTERGY TRANSFER Bh-6G— Sh-B DYE LASSR- P.J. Sebastian' and K. Sathianandan, Laser division, Department of Physics, University of Cochini Cochin - 22. The increasing applications of dye lasers in spectroscopy and photochemistry demand the improvement of efficiency and extension.of spectral region of operation of the dye lasers. The recent investigations in this direction have shown that energy transfer is an effective mechanism for extending the wave length region of la sing in dye lasers 2., An efficient energy transfer process occurs in a dye mixture system when the wavelength region of emission of the donor overlaps the absorption of the acceptor. The tunability range-, the lowest concentration at which the lasing action starts and the power variation with concentration are basic parameters of an energy transfer dye laser, system. We report'here the lasing behavior of a system containing Rh-6G as the donor end Rh-B as the acceptor by 11^ laser pumping. Fig.1 explains the superradiant emission peaks as a function of concentration of. donor and acceptor P ig.1 ( a ) shows the concen tration variation of values for Rh—3 alone while Fi~. 1 (B) max represents the same for donor sensitised Kh-3. It can "oe seen that the tunning range for the ETDL system is approximately the sane as that for nonsensitized system except for a blue shift which is due to an enhanced life time of the. sensitized acceptor £ > J - F i g . 1 ( c ) represents the donor concentration dependence of the emission peak of1the acceptor. At very low concentrations of the donor, the inter molecular distance"is very high and the donor acceptor interaction is very weak. Hence the long wave length ta il of the Rh-6G emission where.Rh-B has no absorption, • X... .is superimposed on the Rh-B emission end a blue shift is observed-. But this blue, shift is less'Compered to that caused by the enhanced life.tim e. When the donor-acceptor concenfrstion approaches 1:1 ratio the energy transfer becomes maximum and the blue shift due to the enhanced life time also becomes maximum. The red shift from the maximum blue shifted wavelength at higher 3h-6G concentration can be explained on the basis of an exciplex formation in the dye mixture W O 650 cavcsfiirmtttm- gf* & - Bh- 8 alone A - Rh-B alose. 1.5 g/l . B - Hh»B 1.5 (ff% * ®h-6G 1 g / l . B - S en sitized Eh-B « G - Kh*B " e * ” 1. 2 5 g /l. e * Se/l Bfe-B * E&-6G 0 - Hh-B • « « ♦ " 2 g / l . 8 - 8h»»S * a ♦ « 8 g /l. - 65 - • ; Pig.2. represents the. superradiant emission spectrum of ' Rh-3 for different donor concentrations along "with that for .unsensitized Rh-B (A). It can be seen that as the donori concen tration increases the intensity of the peak emission wavelength increases, nlien the do nor-acceptor concentration approaches .1:1 ratio the peak intensity be cones the aa zinnia. A> further increase in the donor concentration decreases the peak intensity. This snovs tiiat the linear dependence of intensity on donor concentrations> as predicted by Speiser et.al.{jsj , is valid only at lot: concentrations of the donor. At hi* donor concen trations this linear dependence is "disturbed and shore » decrease in intensity due to conplex formation. *le observed e 200-'? increase in intensity for the sensitised oyster coopered to the unsensitized ayetea. This shows that 27BL systems are more efficient than conventions! dye lasers. CHS 1. Heine rnan end II. Bab row ski, optics Com. 26(1970) Si. 2. Y. kucuooto, luSeto. k. ileiieo end S. Tehixo» Chem. Phys. tetters 53 (1973) 388. 5. P.J". Sebastien and k. Sathianmdca, Optics Cosa. 32 (1930) 422. 4. ?. Crisa and n. rajlyasa, J, Appl. Phye. 47 (1976) 3563. 5. P.J. Sebastian and K. SsifcAaanadan, Optics Com. 35 ( i : do ) u s . 6 . &. speiger sad S. katraro* Op ties Com. 27 (1978) 29-7. - 66 - SOLVENT EFFECT ON AMPLIFIED SPONTANEOUS EMISSION OF ANTHRANILIC ACID. Sharada V. Itagi and Aruna Kulkarni Department of Physics Marathwada U niversity Aurangabad 431004 Berlman has reported on the absorption and fluo rescence spectra of Anthranilic acid in ethanol solution. ( 1 ) . A nthranilic acid shows favourable properties to be a laser dye because of its intense fluorescence, lack of overlap between absorption and emission and strong absorption at the N2 laser wavelength viz. 3 3 7 .1 nm. Meyer et al(2) have observed lasing in the region 398-406 nm for toulene solution of Anthranilic acid. Since they have used a cavity and they have not stated explicitly it is not clear whether anthranilic acid solution in toulene shows amplified spontaneous emission (ASE) or not. Since solubility of Anthm ilic acid in alcohol, acetone and ether were much higher than in toulene, we took up the following investigation to study the solva tion effect on the ASE of anthranilic acid. A nthranilic acid supplied by Riedel was used. The solvents were spectroscopic grade supplied by E. Merck. A rectangular dye cell with canted windows was transversly illum inated by a home made N 2 laser of 300 KW power. A quartz cylindrical lens focussed the No laser radiation to a line of 1 . 2 ? cm length in the cell, spectral distri bution of ASE in different solvents was studied with Carl - Zeiss SH12 monochromator and an EMI 9816Q photom ultiplier. Photomultiplier output was fed to an oscilloscope. The ASE was recorded at regular intervals of 2.6 nm using a bandwidth of approximately 0.15 nm. ASE was weak at lower concentrations in all sol vents. Intense ASE was observed and recorded at a con centration of 5 x 10~3m in alcohol, acetone and diethyl ether. The solubility of Anthranilic acid in toulene being low we could not produce 5 x 10- 3m solution and there was no observable ASE at lower concentrations. The ASE spectrum corrected for instrumental res ponse is given in fig .1. The alcohol solution showed weaker ASE. Acetone and ether solutions showed intense ASE. For acetone solution the ASE is from 382 - 399.5nm (FWHM) with peak at 391.3 nm. The ASE spectrum of ether solution extends from 381 - 401 nm with peak at 389.7 nm. We measured the gain for acetone and alcohol solutions by the method of Shank et al (3). There was no ASE for half the length for alcohol solution and hence gain could not. be measured. - 6? - The unsaturated gain measured for acetone and ether solutions are given in fig.2. For both of them the gain decreases with wavelength and then increases. Since we are using a pulsed source there cannot be appreciable absorption from the triplet state. We are planning further experiments to find out the cause of the dip in the gain coefficient. • Thanks are due to Prof. V.V. Itagi for allowing us to use his N2 laser. One of us (AVK) is grateful to Marathwada University for financial.assistance. REFERENCES 1. Be'rlman I.B. Handbook of Fluorescence Spectra of Aromatic Molecules * Academic Press New York and London (19/1) page 167. 2. Myer J.A ., Itzkan Irving., Kierstead E., Nature(19/0) .225, 5 M f i 3. Shank C.V., Dienes A and silfvast W^T., Applied Physics L etts.(1970) V o l. 1 / , N o . 7, page 307v f* ( a) ^cefone ( b)£it\anol W £rikei*. S c a le . Xaxle . lOoJivn » 1.6 nm. i y axis - lodivn » .1 i i 860 8 8 o 390 ■f'o V/av.elAnjfih C nm.) - 69 - F f j a - ■ . Got Ik Coe^f. <^/ Arflhranllic a d d . i*V~ a) Acetone (coac. SxioSm) b) £.Hher (conc. 3 . 3 * io5m; 5 o S c a L e . x a x is - 10 divn. r 2 .6 nos. 0 20 y anjs - 10 divn. = S . 3 8 0 3 9 0 10 Wove! englh CA"0 - 70 - ANT 1ST OSES FLUORESCENCE IN NEUTRAL RED EXCITED BY 632 .8 ran He-He LASER. Sharada V. Itagi and Aruna Kulkami Department of Physics •Marathwada U niversity Aurangabad 431004 Orange fluorescence is observed when a. solution of neutral red in gl'ycerol is exposed to the radiation of high pressure quartz mercury arc. The same solution when irradiated by 6 3 2 .8 nm radiation shows fluorescence but shifted towards red. Since.632 .8 ran radiation is beyond the strong absorption range of neutral red we have carried out detailed studies of the fluorescence and associated absorption of 6 3 2 .8 ran radiation by neutral red. •, The experimental set up is given in fig.1. The fluorescence spectra excited by He-Ne laser and by broad band quartz mercury arc and the absorption spectrum are" given in fig . 2 . It is seen that the absorption of neutral- red at 632.8 ran is weak and the fluorescence excited by 632.8 nm is on longer wavelength side of that excited by quartz mercury, arc. One likely process responsible for the fluores cence could be two photon absorption. The laser power is low and the fluorescence intensity measured at various attenuated incident intensities shows linear dependance and thus a single photon process is established. The likely single photon processes that could produce observed luminescence are ( i) dimer fluorescence (ii) phosphorescence (iii) antistokes fluorescence. In the range of concentrations studied viz. 1.2x - 71 - 1 0 ~^ t o 3 x 10 -^ M no spectral shift was observed and hence the dimers do not appear to play any role in the fluorescence shift. ( 1 ) since the efficiency of fluorescence is comparable for laser and quartz mercury arc excitations the long wavelength shift could not be due to phosphorescence". The luminescence may be antistokes fluorescence for which the absorption from higher levels of the ground state is responsible.• ( 2 ) The antistokes fluorescence is found to be red shifted in many molecules. (3) The absorption from higher vibrational levels of monomers, should increase exponentially with temperature. To test this we carried out further experiments. A pyrex absor ption cell 7.5 cm in length and 1 cm inner bore was heated electrically. The cell wall temperature was. measured by a calibrated copper constantan thermocouple.■ The tempera ture was varied from 3 0 0 °K to 400°K. Spectra physics model 404 power meter was used to measure the absorption of 632.8 nm radiation by neutral red^ solutions. The varia tion in the absorption at different temperatures was measured for number of concentrations. Fig.3 shows In al .vs 1/T for two typical concentrations. The initial increase indicates that the absorption from higher • vibrational^states gives rise to antistokes fluorescence. The decrease in absorption at higher temperatures may be due to a shift in the absorption spectrum due to a change in solute-solvent interaction with temperature. Since the dielectric constant of glycerol decreases with temp erature the shift may be of this type.(4) One of. us (AVK) is grateful to Marathwada Univer sity for financial assistance. - 72 - _chof>)sef Fig. i . XxJ>er1i^eKh»l arrangement". Fig.2_tlei#l>eFred sol*, i* Glycerol- - ..cone. 305 * idr M oles/lit . a) Absor|btioi\ curve. b) tioF»*al' Fluorescence edited by M e rc u ry . c) Fluorescence under 652.8 nm. 440 910 600 680 - 73 - .303 x 10 'iif. .1.5 .201 * 10 moies/lit- 3.5 2.5 3.0 Flg.3.Variation of Ln«I w'tfh temperature oj Weutral red \ Glycerol. REFERENCES 1. Parker C.A. and Hatchard C.G.; Trans.Faraday Soc. 59,(1963) 284. 2. J a i n R.K-. , Hu C ., G u sta fso n T .K ., E l l i o t , s .S . and Chang M.S.; J SPECTRAL EVOLUTION OF A N2 LASER PU PIPED CRATING TUNED DYE LASER K. Das gup ta , Fl.O.R.S., BARC and L.G. Halt, Laser Section, BARC INTRODUCTION: There has been considerable th eoretical and experimental interest in the .temporal evolution of the spectrum of puleed Dye Lasers - (1) - (5) . Such interest has been stimulated by a need to understand the operations of Dye Lasers and Amplifiers under various types of pulsad excitation. Experimental studies reported have been made mostly with Flash lamp pumped and Ruby or 3+ Nd : Yag la ser pumped broad band Dye Lasers. Observations and analysis have been concerned primarily with the following aspects of the spectre temporal evolution of the Dye Laser pulse: (i) Shift in wavelength of the peak of .the instantaneous laser output spectrum during the evolution of the laser pulse... (ii) Progressive spectral narrowing of the laser output. The sp ectral width measurement techniques for pulsed dye lasers in general determine the time Integrated line width. This paper taperta the observation of apsctro-temporal evolution of a N2 laser pumped Dye laser with a grating as a feedback element for tuning and norrowing - of the output. .. = 75 - EXPERIMENT; The experimental s e t up for measurement of instantaneous spectrum i s shown in fig. 1. The oscillator is a transversely pumped -3 5 x 10 Molar solution of Rh6G in ethanol in a 10 cm dye cuvette with a 30% output mirror and a 1600 lines/m o grating in littro w setting as the other feedback elements. The line width, of the e la ser was measured to be ~ 40 A. A email fraction of the ^ laser is used to trigger the oscilloscope through a vacuum photodiode; the laser waveform in the CRO is used as a time reference and , also for monitoring the laser power. The dye laser is given an optical delay larger than the ^ laser duration and recorded through the same photodiode on the storage oscilloscope after being sampled by a 3arell-Ash £ ra Monochromator (Instrument band pass ~ 0.8 A ) . The waveforms of the different wavelength contributions sampled by the monochromator are superposed (fig. 2) and the contributions at different times determined. This gives the Dye Laser spectrum at different times starting from the onset of lasing (fig. 3), from which both fblHM and peak wavelength can be determined. RESULTS; The results show a small shift (fig. 3) in wavelength of the peak of the instantaneous spectrum as the laser evolves and a progressive narrowing of the FUHM (fig. 4) which reaches a constant value after 5 to 6 ns. The spectrally integrated pulse waveform (fig.5) indicates the stage of laser evolution at which the FTdHfl becomes constant. n laser j,*' « 2 FIG.1. EXPERIMENTAL ARRANGE MENT , C. L.-CYLINDRICAL LENS, DYE LASER 6RATIN8 B.S. BEAM SPLITTER, M ,M l. MIRRORS, MON-MONOCHROMATOR □ TUNING PD.-PHOTODIODE. DYE CELL TEK 7834 P D. o DYE LASER MON. C.R.Q FIG.2. SUPERPOSED'OSCILLOSCOPE TRACES SHOWING DIFFERENT WAVELENGTH CONTRIBUTIONS. N2 LASER IS USED FOR TIME REFERENCE. 1-5864 ; ' 2-5870, 3-5874, 4-5876; 5-6882, - 6-5886,7-5890.8-5892 IN A. in 10 8 9 10 TIME IN ns- - 77 - 2 60 50 <$20 10 hs ' 5864 5874 4 5884 5894 5899 w a ve le n g th IN FIG. 3. INSTANTANEOUS SPECTRA OF DYE LASER OUTPUT. © Q ® O 2 4 6 8 10 0 2 4 1 6 8 TIME IN " s — TIME IN ns FIG. 4. PROGRESSIVE SPECTRAL FIG.5. OSCILLOSCOPE TRACE OF VWELENGTH NARROWING IN TIME INTEGRATED. DYE LASER PULSE .WAVEFORM- References; (1) F.P. Schafer, Ed. Oye Lasers. Selint Sprlnger-Vetlag, 1973 (2) V.H. flayer and P. Flaoant, "A basic property of Oye Lasersi Spdotral evolution", Opt. Coen., _19, pp 20-24, October 1976. (3) 8.S. Neporent and V.B. Shilov, "Spectral shifts in thelaaer emission of laser pumped dye solutions", 0pt«Spectrosc., 30, pp. 576-579, 1971 (4) L. Singer, Z. Singer and S. Kimel, "Wavelength s h if t s in tuning dye lasers", Appl.Opt., 15. pp 2678-2683, Nov. 1976 (5) fl. Base and 3 .1 . S tein feld , "Wavelength dependent time develop ment of the intensity of dye solution laser", IEEE 3. Quantum. Electron •» i.* pp. 53—58, Feb., 1968. - 78 - A DOUBLE-PULSED RUBY LASER BOB'PULSED HOLOGRAPHY Rama Chari, G. Chakrapani, Bh.A.R.B. Raju, K.R. Sanaa an d Putcha Venkateswarlu Indian Institute of Technology Kanpur This paper discusses the design features and performance of a Double Q-switched ruby laser which pro duces two gaint pulses of individually variable peak powers and variable separation between them. The laser operates in a single longitudinal and Transverse (TBMqo) modes. Each of the two pulses contains a maximum energy of 100 to 150 mJ for a duration of 25 nsecs (approx.). Lasers with these specifications are needed for double pulsed holographic interferometry. Fabrication of this laser has been taken up by IIT Kanpur on the request from M/s. Bharat Heavy E lectricals Ltd.(Hyderabad) for their stress analysis studies of rotating system. The optical Train assembly for this laser is shown in the accompanying diagram. A 5/811 x 311 Ruby rod - 0.05 percent C^O^ concentration by wt. , AR coated end faces is pumped by Helical flash lamp (EG and G - FX 600) arranged in a cylindrical pumping cavity. Helical flash lamp is specifically chosen to improve the pumping geometry which enhances the TEffiQo mode output. •The whole assembly is cooled by submerging in distilled/D e-Ionized water, cooled to 15 to 20 C by circulating in "a.closed- cycle through a refrigerating system. The laser rod holders are designed to make a.water tight seal with minimum strain on the rod. Further, to ensure that al most the entire length of the Ruby rod.(greater than 90 percent) is pumped and to minimise contamination to the rod the later is enclosed in a suitable glass sleeve. This glass sleeve further assists in filtering any UV content of the lamp output. Provision is also made for - 79 - Q_ P.C Delay . Fus K Lamb ^ Pulse a - i ° . A r - k - - \ X-BuUfc. E Pulse N j x X - 80 - seeping, tne laser: rod in a dry/clean atmosphere by in troducing dry nitrogen/air from a standard gas cylinder into the glass sleeve to flush the faces and body of the Ruby Rod. This also assists the laser to be- opera ted below the ambient temperature without any dew forma tion on the body and faces of the rod. It is to be noted that low temperature operation assists the achie vement of both longer coherence length and lower opera ting thresholds for the laser. The input to the flash lamp is supplied by discharging a capacitor bank of 560 ,uF chargeable to any voltage in between 1-4 ICV. The flash lamp trigger ing is being achieved by using an EG and G (Model TS 146) series - triggering transformer. The secondary (saturable) inductance of the transformer stretches the pumping pulse to approximately 1 msec., duration. Longi tudinal mode selection has been achieved by using a 3- plate Resonant reflector of nominal 60 percent reflec tivity. The output coupler has a reflectivity of 60 percent (or 80 percent for low output power with low threshold), an intracavity aperture (pin hole of vari able size) encourages laser operation in the fundamental (TEM ) mode. 0 0 A KD P Pockels cell (Lasermetrics Model 812 PV) operated in 'pulse-on' mode in conjunction with a Glan-Tylar polariser Q-switches the laser. The 'half wave' voltage pulse required for this purpose is obtai- hed from a Krytron switched drive electronics. As shown schematically in the diagram, the drive-electronics generates two (^-switching voltage pulses of variable amplitude to drive the pockels cell. The timing of these pulses can be independently delayed (in steps of 1 gsec) w .r.t. flash lamp Triggering pulse upto a maxi mum of one millisecond. This enables the Q switching - 81 - and hence the laser operation to be delayed w. r.t.flash . lamp firing and also w. r. t.' each other by a maximum of one millisecond, thereby generating double ^switched laser pulses. By adjusting the amplitude of these high voltage pulses (independently) the peak power in the two pulses can be adjusted to be equal at all output levels of the laser. Both manual and Auto mode of operation for the laser is incorporated and the laser can be fired at a maximum repetition rate of 2 pulses per minute. More details about the performance characteristics of the laser will be presented with the help of slides at the sym posium . Jycknowledxements : The authors gratefully acknowledge B rs. DrR. Rao, O.K. La-1, M. K. I h e e r , S. Jam au r , D. liadhavan, U.V. Kumar and h. Jagannath for their involve ment in discussions at various stages. The help' rende red by our technical staff in Physics and Electrical Engineering Departments and in our Central Workshop is highly appreciated. Finally the work presented here was made possible due to the financial help given by BHE1 (Hyderabad). Ref erences 1. Solid state Laser Engineering : W. Koechner (Springer- Verlagj New York, 1976). 2. TRW Instruments data sheets on their Ruby laser sy ste m 3. Operation and service manual-Model 22HB-Double Pulsed Holographic Laser System, Apollo Lasers Inc., California. - 82 - Study o f Thermal Lensing In a High Power NdiGlass Am plifier Psing a ffsyefront Shearing Interferometer B.J.Dhareshwar, T.P.3. Nathan •, J.S.Uppal & B.L.Gupta laser Section Bhabha Atomic Research Centre Bombay-400085 It has been observed * * that a high power laser amplifier rod is subjected to a thermal radial gradient during the flash lamp pulse, leading to the formation of a thermal lens. This thermal lens is time dependent. It has also been reported that an amplifier pumped by linear flash lamps closely surrounding it behaves as a 4 5 weak negative lens during the pump pulse . This would consequently lead to an increase in laser beam divergence. Therefore, in the staging of a high power laser it is important to know the focal length of this transient thermal lens. Several methods 8’"^’8 have been used to determine the thermal lens focal len g th . Most of them are interferom etric methods where excellent stability of optical components is needed. We suggest below a method which has the following advantages: (l) Measurements are made at 1.06 fx using a pulsed laser probe beam. Hence effects on the refractive index due to the inverse aiomalous dispersion at 1.06yu a, however small, will be accounted. (2) A shorter exposure leading to a better temporal resolution ( 5 ) simplicity. A parallel plate 9 lateral shear interferometer devised by Murty is used. The laser probe beam is incident on this plate at a small angle. The two beams reflected from the front and back surface of the plate are laterally sheared parallel to x-direction due to the thickness of the plate. The shear 3= t Sin2i (y^-sin^i) 3 where t is the plate thickness, is the refractive index and i the angle of incidence. For a gaussian beam of field distribution £■» So _c fkl + h ^) - b n L J r L i Z U Z Z f r ' ) 21 The phase difference between the beams reflected from the front and the back surf ace c. is A (p - k'*-5 . 2-k.vvhcos© _ .. i G + i w j where O is the angle of refraction inside medium. - 83 Separation between two bright fringes will then be d, =r Vfe U +^o/g) S • . At large distances from the beam waisfc 25>2y and ~zL 'a 'R C i) , hence the radius of curvature of the wavefront at z is given by G L = \ K ( * ) j s or '£»(’*)» ^ 5/> If aruV^iCi) 816 *b® radii of curvature of the wavefront measured after the amplifier, with and without pumping respectively, then, the focal length of the negative thermal lens formed is = 4 © ' Experiment: Thermal lhnsing ih a 19 mm^x 300 mm NdsSlass rod was studied. Six linear tenon flash lamps in a close circular diffused reflector cavity pumped the amplifier rod - Fig.l(a). The flash lamp pulse temporal profile is shown in fig.1(6). The electrical input to flash lamps was between 6-10 KJ. The experimental set up is shown in fig. 1(b). . The TE2S laser probe beam from a Hd:YAG oscillator with a beam divergence of 1.5 mrad was allowed to expand naturally to fill thti amplifier rod (spot size at amplifier was 12 mm). After propagat ing through the amplifier, this beam was incident on the shear plate (2" x 2" block of thickness 2.54 cms) at 5®» The arrival of-the probe pulse at amplifier was 460 nsdcs after the ataft of the pump pulse. The sheared wavefronts were incident on an infrared image converter tubd (BARC/D-2201). The output was photographed with a 35 mm camera oh a 400 ASA film. Photographs of the fringe pattern taken with and without pumping amplifier (9.5 KJ electrical input) are shown in fig.2 . The scans of the fringe patterns were taken on a microdensi tometer to determine the fringe spacing accurately. Fig *3 is the plot of j - vs energy input to flash lamps. Conclusions: A simple and elegant method to determine thermal lensing in high power amplifiers is suggested. The linear graph of -f vs in- :put energy confirms the fact that the rise in temperature at any position along the rod radius is proportional to the energy input to flash lamps1*. in fact the transient lens action can be studied at various times during the pump pulse by varying the delay between probe pulse and flash lamp pulse. The straight fringes in the interferogram in d icate no d isto rtio n of the spherical wavefront during pumping. - 84 - Referesoeat 1. W.Koechner, Appl. Opt. 9 , 2548 ( 197Q) 2. 0.3.Baldwin, E.P.Biedel, 3 . of Appl. Phys, 38, Z726 (1967) 3 . P..L.Townsend e t a l, Appl. Phy. L e tt., 7, 94 (1965) 4. U.K.Chun and J.T .B iskoff, IEEE J.Q.B. 7, 200 (19?1) 5. B.ClB'oraham, Appl. Opt. 9, 1727 (1970) 5. J.K.Bradford et al, Appl. Opt., 7, 2418 (1968) . 7. 5.D.Sims et a l, Appl .Opt. 5, 621 (1966) 8. A.Y.Cabezas, et al, Appl.Opt. 647 (1966) 9. X.V.B.K.Murty, Appl. Opts. 3 , 531 (1964) 10. J.B.Saunders, J.Bes. Hat. Bur Stand, 65B, 239 (1961) ,11. W.Koechner, 'Thermal effects In laser rods', 366, 'Solid State Laser Engineering', Springer Verlag (1976). - 85 - Ancdiiea Alvhiwivh %£FLEcrroJt. V jA rsR TAckET Q Flash lamps 3> AHPLf FIE< Roo . 0 0 *(0O 4« 600 9» /cco e -* ""r,NEY/**6eCS) CCJ AHPL/F/cA VbvwsEF/ioNT ( a ) l^snm fooH csciujvron - la k v 3Snvn * CAMERA S«£AREj> ILtTERPE RObRAM Cb) F / 6 - - 1 loo O A. 4 6 « io »<*. E le c tric al V w p u t e n e r g y - * 1*3") • C b ) F /C r-2 . 85 Transient Lena Measurement In Optically Pumped Claaa Amplifier T.P.5.Nathan, J.S.Uppal, L.J.Dhareshwar, B.S.Narayen & D.D.Bhawalkar laser Section Bhabha Atomic Research Centre Bombay-400085 It is well knoton that an optically pumped Nd: glass laser amplifier behaves as a wfeaK negative lens'. The lens action arises due to non- uniform pump light absorption resulting in a radial temperature gradient. Several techniques have been reported for the measurement of this time resolVed lens formation. Interferometrie techniques coupled w ith a high sptiefl fTamihg camera^’ have been used. A simple technique using a dominated He-Re probe beam and an external long focal length converging lefas has also been used^. In this method, actual shift of .^aussiah probe beam waist gives the idea of lens action at any irlataht of pump pulse. The disadvantage of this technique is th a t i t requires a large number of shots. We propose a simple method of thermal lensing study in optically pumped amplifiers* The advantage of this technique is that it gives a time resolved lens foitiation in a single shot. The experimental set up for our experiment is shown in fig. 1. A collimated TEMo<) laser beam (13.2 dm diameter at l/e points) is used as a probd be ami The central portion of the beam of 6 mm ! dia is selected by using a hard aperture and the light is detected by a PIN diode. On Optical pumping, the laser amplifier behaves like a negative lens dilt-ing the pump pulseyso that beam divergence ihcreasea and the power passing through the fixed aperture decreases. Knowing the o p tic eU. signal transm itted by the aperture w ith and with out firing the amplifier,one can calculate focal length of the medium; For a Gaussian beam with parameter ZQ falling on a lens of focal length F, placed at a distance d1 from the waist of the beam,the spot siv.es at distance dg from the lens are related as^ in fig.2) - 87 - where UJ n and ij ^ are the spot radii (1/e^ intensity point) with and without lensing respectively. In order to find cut focal length,enath. one needs to know ./,Ij i m , and beam oarameter 2 . o For a G aussian beam whose intensity distribution is given as: _ I O ) - I 0 e - x p — £ 9 ^ 0 3 OJ‘ where II) is the spot radius at beam waist, pcwer transmitted by an aperture of radius ' a9 is: Tcu - J X 0 exyb ( - 3 ^ ) S-TT^dH (*) = - K r l t - . / - 2 : = f4-J 7>r w -h J Knowing the fractional power P^/P^ transmitted by aperture, one can find out UJ and UJ for the beam, m n Beam parameter z?c and position of beam waist is calculated by finding the curvature of wavefront using a shear interferometer, and spatial scan of the beam by using a thin slit. The spot size UJ at a distance Z from beam waist is related to radius of curvature R of the beam as: 2 = o C ^ F ? (5) where y _ UJ^M »a ^ = X + (6i Knowing 1Z from 63/71 (5 ), ZQ is calculated from equation (6 ). Laser amplifier uses a 19 mm diameter, 30 cm long, 3.1% doped LSG-91 H Kdsglass rod. A saturated solution of sodium nitrite is used as a coolant. Laser rod is pumped by six linear xenon flash lamps in a close coupled anodized aluminium cavity. For our experi ment values of various parameters are given as: 32 A/S — LJci,ry-Ou) ^Om'cL ' f 'clte.-r L- fe e t i s s i L e m s % 1 ,-Td^ - J o y A tf C /?° " Oscc'tLoscape P lG r-l - 89 - 4 .0 HV <7^ ‘i Kzz> V) -t0 •3.0 kv C.4-0 Hj j X^Cask^cumja Javlse 4co 400 £oc g»o /a,® /^ro <^o /6t» Z6ec ^oco ^ (prrtioiasecjoyxds^ 'Wyv iVtA r^T Fl& .', 2, A £• \m 3 1 ° ^•oKv (4-oK^J y = e e v s c 24 0 .okvtn-2, KTJ £00 o 1 fio d F I& 4- Cm acr-65eco'« cts). Por Fig 5. Please see plate C at the end of this book. QUANTUM OPTICS 1. Exact thermodynamic behaviour of an off- 91 resonantly driven Dicke model - S.V.lavande and R.R.Puri 2. Phase transitions in the driven Dicke 95 model - E.E.Puri and 3.V.Lsvande 3* Quantum phase transition in a Dicke 59 System of H harmonic O scillators distributed uniformly over an arbitrarily large volume and radiation in thermal equilibrium - N.Chandra and R.Prskash 4. Theory of Optical HAtJLE effect 102 -P.Anantha Laxmi and G.S.Agarwal 5* Nonlinear Transient and Steady-state 106 response in a classical model - M.D1souza, R.D'souza and A.Kumar. 6. Photon echoes generated by three 109 excitation pulses - H.Prakash and G.S.Bhatnagar. 7. Laser induced Eabi-frequency quantum 112 beats in superflourescence - H.Prakash and N.Chandra. 8. Steady propagation of a single pulse 115 in two—photon resonant medium-second harmonic resonance. H.Prakash and G.S-Bhatnagar. 9. Coherent oscillations of atomic beam 116 - J.B.M istry and R.D'souza. - 91 - EXACT T H£K'ODYNAi-IC 3BHA710UB OS1 AS 0?F -RESOliAHTL Y DRIVES DICKS .VCD S.V.Lawande and R.R.Puri Theoretical Reactor Physics Section Ehabha Atomic Research Centre, Troabay, 3oabay-400 085 A system of N identical two-level atoms confined to a volume of dimension less than the radiation wave-length (The Dicke model1) and resonantly driven by a coherent laser field is known to exhibit interesting co-operative phenomena like the collective resonance fluorescence^ and optical bistability?. Such a svsten is usually described by the superradiant master equation^>5 with an added term to describe the action of the external field. Recently, an exact steady state atomic density operator corresponding to this master equation, valid for arbitrary K and normalized Rabi frequency /g/ has been obtained by us°. In the thermodynamic lim it N — > 0 0 with 2/g/l! = 0 = constant, the derivatives cf the atomic observables showed a discontinuous behaviour at £? - / , reminiscent of_.a second order type phase transition0-9. The solution of Ref3 has been extended further to the off-resonantly driven Dicke model1®. ■The present paper deals with the thermodynamic behaviour of this model. The master equation for the off-resonantly driven Dicke model in rotating wave and dipole approximations and in the rotating frame of the driven field reads as -= - L JTL [S + + Ca ] i~ L L^Z ; > 7 ct t V J „ / -2 S_ (a $+ - S-b S- J . A Here (^ is the reduced atomic density operator, 2-0. is the laser Rabi frequency, ~ { L0-- to,-.) i s t i)e frequency detuning between the laser and the atoms and 2 i'0 i s th e E in s te in A- coefficient. The collective atomic dipole operators together with the atomic population inversion operator £ ^ s a tis f y the usual angular momentum commutation relations. The steady state solution ) s s C obtained by setting d 0) is given by 0 v c C C ~ 92 - X ;„= J-L f^* rm s-)7 sX m rn.= O n — 0 where ^ — c — i y / o j ^ ^ ° y /3,(i while y - / i'i l — Zi r ( r~L -rh xl c hx ii j n j r^yZ~^J~ /V £ > r= 2 - a /V/v ^ i d r * ,n ~ f~> L~ O ^ (3) , __ A / -A ' ' i i-O.1 Cr ,~1 ■) 2 >'1 ( X / - />l) / (J /,2 V-/) / The exact expression for (^A)ss is employed to obtain the steady state expectation values for the atomic operators, the atomic fluctuations and the atomic correlation functions^® for arbitrary N. The thermodynamic behaviour is obtained from the asymptotic expression for D in the limit of large if given by11 D ~ V i V H 7> where & — ■ 2 I % I N ~ 1 j = Z f & ) N ~ ! and Vo "$)■=) ~ J (Z jJ ^ --£ <£x ±-iz » ) A > f j (5) This expression is used to obtain the asymptotic forms for the atomic observables tVL x , tn.^t and . Thus - 93 A/_^ 00 z ^ (6) my — 9/at/t-z3-) 2 ma ~. '~ OO -tiO ' lt-_s "It follows that ftlx , rny t and ^ j and their derivatives with respect to ,£ are ‘continuous functions of & ^ ^ <=0 / for all non-zero values of *o as required for the validity of the factorization ansatz. This behaviour is in contrast with the resonance case (^y>~o) where as N — ° (& < 0 while beyond Q = /, O e Fig.1. Fluctuations in the steady state value of the atomic population inversion for Acknowledgements The authors wish to thank Dr.S.S.Hassan, Department of '-.'.athematics, vRitoT, i-aachester, England for useful correspondence. References 1. H.iT.Dicke, Phys.Rev. 52* 99 0 954) 2. G.S. Agarvial, A.C.Brown, L.M.Marducci and G.Vetri, Phys.Rev. Aljj, 1615 (1977) 5. E.Bonifacio and L.A.Lugiato, Opt.Comm. 1_2, 172, Phya.Rev.Lett. 40, 1025 0973) 4. 0.3.Agarwal, Phys.Rev. A2, 2053 (1970) 5. R.Bonifacio, ?.Schv/endimann and F.Haake, Phys.Rev. A£, 502 & 354 (1971) 6 . 2.R.Puri and S.V.Latvande, Phys.Lett. 72A. 200(1979); ■Baysica 101A,599 (1983) 7. L..i .Harducci, D.H.Feng, R.Gilmore and G. S.Agarwal, Phys.Rev. A1J3, 1571 (1973) 3. F.D.Drunmond and H.J.Carmichael, Opt.Comm. 2J_, 160 (1973) 9. S.S.Hassan, R.K.Bullough, R.R.Furi and S.7.Lawande, ■ Phyaica 105A. 215 09SO) 10. It.S.Puri, 3.V.Lawande and S.S.Hassan, (to appear) Opt.Come. (19S0) 11. 3.7. Lawande., S.S.Hassan, R.R.Puri and R.K.Bullough, (to be published) - 95 - PHASE TRANSITION IN THE DRIVEN DICKS MODEL R.R.Puri.and S,7.Lawande • Theoretical Reactor Physics Section Biatiia Atomic Research Centre, Tromhay, Bombay-400 085 The driven Dicke model which considers N identical two- level atoms on a single site collectively interacting with an imposed CW resonant laser field is of great interest in quantum optics. It has been used to discuss the phenomena of optical bistability2 and co-operative . esonance fluorescence**4, In the case of optical bistability, the atomic observables show a first order "phase transition" resulting from a competition between the independent atomic decay and co-operative decay mechanisms2,5. However, when only the co-operative decay is considered the resulting phenomenon of resonance fluorescence involves a second-order "phase transition" . Thus the model provides an interesting example of a system exhibiting co-operative phenomena far from thermodynamic equilibrium^10 . In this paper, an attempt is made to draw an analogy between this subtle continuous phase transition and the usual thermodynamic phase transitions on the basis of some exact r e sults'^2. For this purpose, it is convenient to consider the equations of motion for the off-resonantly driven Dicke model which can be expressed, in dipole and rotating wave approximation, as = <(s+szy -2 j^ s zy (1) where H — , A . — and c . The operators S S z » are the usual collective atomic oper 2-CX. is the laser Rabi frequency, 2. / o is the Einstein A-coefficient and £ 0 = (c d - the frequency detuning between - 96 - the laser and the atoms. For the exact steady state analysis1? show that the factorization ansatz <0 S y ? / / \ ‘:l — a/ a is . valid in the thermodynamic limit-/V— yoO. leaking this ansatz in Eqs,(1) the atomic population inversion — ob'tained as - ^ 4 (2) where > 4- - / with d? = 2/g/H~ and ^ — ■ ? . / /V aa the scaled Rabi frequency and the scaled detuning parameter respectively. In particular, it is seen that at resonance ^ A L , ;= 5= J 2- S'-# 2- 0< / (3) = O > / which is the result obtained in the exact quantum theory in the thermodynamic limit /y— 7»®. Thus has two branches for <5 Returning to the off-resonance Dicke model, it is seen from Eq.(2 ) that for all non-zero values of (fi , both and its derivative aire continuous with respect to 0 2 . The critical behaviour disappears due to the off-resonance dispersive effects. The detuning parameter • One may then define the specific heat £ ^ ±.)p. ^-^/"j^.and the susceptibility X y - ^ - X 0 ~ e^O-e2! 3^ e< 1 W Thus the critical exponents are &=. , = The analogy may be stretched further by defining Gibbs free energy G C h 'l-2 j & x ) aB g — Go y x rrlz — /i7z 3 . (5) This form is consistent with the fact that its minimum ( ~ ° ■■ ^ ^ o j indeed yields the stable branch hl2 ~ — 'p /z . Precisely, because the order parameter had only one stable branch, the expression for the Gibbs free energy G is a cubic rather than the Landau farm involving fourth power in the order parameter. It is clear that the behaviour at resonance (fj -= 0 also follows from the form (5) of G. An alternative form for G has been discussed by Hassan and Bullough*4. References 1. R.H.Dicke, Phys.Rev. ^L, 99 (1954) 2. D.F.V/alls, P.D.Drummond, S.S.Hassan and H.J.Carmichael, Prog.Theoret.Phys.Suppl. 6^, 507 (1973) 3. R.Senitzky, Phys.Rev. A6, 1171» 1175» 0972) ; Phys.Rev.Lett. 4 0 , 1334 0 973) 4 . G.S.Agarwal, A.C.Brown, L.M.Narducci and G.Vetri, Phys.Rev. A1J, 1613 (1977) 98 - 5. G.S.Agarwal, L.M.Narducci, D.H.Feng and H.Gilmore, Phys.Rev. ATS, 620 (1978) 6. L.M.Harducci, D.H.Feng, R.Gilmore and G.S.Agarwal, Phys. Rev. A18. 1571 (1978) 7. P.D.Drummond and H.J.Carmichael, Opt.Comm. 27. .160 (1978) 8. R.R.Puri and S.V.Lawande, Phys.Lett. 72A . 200 (1979); Physica 101 A,. 599 (1980) 9. H.Kaken, "Synergetics" (Springer -Verlag. 1977) 10. G.Nicolis and I.Prigogine, "Self Organization in non- Equilibrium Systems", (Wiley, 1977) 11. S;S.Hassan, R.K.Bullough, R.R.Puri and S.V.Lawande, Physica 1.03A, 213 (1980) 12. R.R.Puri, S.V.Lawande and S.S.Hassan, (to appear) Opt. Comm. (1980) 13. S.V.Lawande, S.S.Hassan, R.R.Puri and R.K.Bullough (to be published); also, S.V.Lawande and R.R.Puri (This symposium). 1 4 . S.S.Hassan and R.K.Bullough,(international.Conference and workshop on Optical Bistability Asheville, N.C. USA (June 1980) QUANTUM PHASE TRANSITION IN' A DICKE SYSTEM OF N HARMONIC OSCILLATORS DISTRIBUTED UNIFORMLY OVER AN ARBITRARILY LARGE VOLUME AND RADIATION IN THERMAL EQUILIBRIUM NARESH CHANDRA and RANJANA PRAKASH Department or Physics, University or Allaha-bad A lla h a b a d Hepp and Lieb1 studied a Dicke system of N tv/o level atoms and a single mode radiation in a small en closure in thermal equilibrium* and predicted a phase transition, to superradiant state, below a critical teap- 2 nature. Wang and HLoe derived the same results in an elementary way and generalised this treatment to include finitely., many: modes of radiation. These authors left out of. consideration the. counter-rotating and the A^ terras in the Hamiltonian,- Hioe' and Carmichael, eardiner and 4 Walls included the counter-rotating terms and obtained phase-transition, but. at a different critical te me nature. Mallory-' showed that the condition for phase-transition holds at such, large atomic number densities that, the Dicke1s assumption^ that the atomic wave functions do not. overlap does not hold good.-Rgazewski, Wodkiewicz 7 and Zakowi.cz later found that the phase-transition. is obtained when an incomplete Hamiltonian, is used and the A^ terms or. the counter-rotating terms are neglected... This problem has been studied by many authors1"^ >I“ 9 a n g i t is now realized? > 9 that no phase-transit.ion takes place in this system.. The parallel problem, involving a Dicke system - 100 - of N-’narmonic. osillat-ors distributed uniformally in. an. arbitrarily large volume and interacting with, radiation field has received, no attention, This proplem is discussed in. this paper. The Kamiltonian E for the above mentioned, system is diagonalised in two coupled-radlation-oscllator modes and IT — 1 coupled oscillator modes. It is shown that the a2 -term in H: cannot, be left, out of consideration; if it is left t the quantum, of excitation, energy in one of these coupled-radi.t.ion-oscillator modes may become negative the energy of the system then has no lower bound and the Boltzman factor exp(-fi/lcgT), where kg i s the Boltzman constant and T is the absolute temperature, becomes un bounded, The partition function and the free energy of tte system are evaluated. These quantities and all their derivatives appear as 'continuous functions of. tempratur.e and rule out a phase-tr.ansiti.on. in the usual thermdynamic s e n s e . • The complete.and. reduced density operators are found and the quasi-probability functions are seen to be generalized Gaussian, For every value of the density of oscillators, a critical- tempraturer. exists below which the system makes transition, to a quantum phase in which the weight functions of diagonal phase-space representa tions of the reduced density operators are not-positive- d e i f i n i t e , It is also shown., that the quantum phase transit ion also occurs in the generalized case of finitely many - 101 - modes of radiation. An easy, completely quantum derivat-s. ion of the dispersion relation for propagation of light in a media of such oscillators is also given. * References: 1. K. Hepp and E.H. Lieb, Ann. Phys. _76, 3 60 (1973)- 2. Y.K. ffang.and F.T. Hioe, Phys. Rev. A7, 831 (1973). 3. F.T. Rioe, Phys. Rev. A8, 14-40 (1973). 4. H.J. Carmichael et al, Phys. Lett. 4_6A, 47 (1973). 5. 7/.R. Mallory, Phys. Rev. Al i. 1088 (1975). 6. R.H. Dicke, Phys. Rev. 23., 99 (1954). 7. $C. Rzazev/sky et al, Phys. Rev. Lett. 3_5, 432 (1975); Phys. uett. A58, 211 (1976). R. Gilmore, Phys. Lett. A55, 459 (1 9 7 6 ). 8. Y.A. Kudenko et al, Phys. Lett. 50A, +11 (1975). 3.M. Pimentael and'A.H. zimerman, phys. Lett. 53_A, 200 (197 5). R. Gilmore and C.M. Bowden, Phys. Rev. Ai3, 1898 (1976). V.T. Emeljanov and Yu.L. Klimontovich, Phys. Lett. A59, 366 (1976). B.V. Thomson, J. Phys. A10, 89 (1977). 9. K. Rzazewsky and K. wodkiev/icz, Phys. Rev. A]_3, 1967 (1976). v. M. Knight et al, Phys. Rev. Ai7, 1454 (1978). I. Bialynicki-Birula and K. Rzazewsky, Phys. Rev. Al£, 301 (1979). - 102 — THEORY OF OPTICAL HAMLE EFFECT P.Anantha Lakahml and 0.3. Agarwal School of Physics» University of Hyderabad, Hyderabad The Kanle effect finds wide applications in the study of the lifetim es of excited states and the measurement of various relaxation and collisional parameters*. In the normal Hanle effect, the system is prepared in a coherent superposition of the Zeeman sublevels and their properties are studied by observing the fluorescence as a function pf the magnetic field. The linewidth of this fluorescence signal depends critically on the nature of excitation. ' A novel variation2 of the Hanle experiment involves the use of a suitably polarized strong off-resonant laser, in addition to the pump laser, which lifts the degeneracy of the excited states and. shifts each level by a different amount due to light shifts. The fluorescence is now obser ved as a function of the intensity or frequency of the strong off-resonant field. In this case, Kaftandjian at al2. found that the zero field level crossings occur in a sim ilar manner as in the magnetic field Hanle effect. Here we develop a gene H# ral theory of "the optical u / Hanle effect which is valid iA .J U .ii for arbitrary values of the [ i it *7 strength and bandwidth of /* ...... •* the pump field . We have con Jfi___ L------1 sidered the Hanle tranei- m ■-W t i o n s ta k in g p la c e b etw e e n iaroi»«i i»opueu iimi, eitwt, the levels with angular mo mentum J=0 and J=1 respecti— _ . vely. A schematic diagram of ty the energy levels is shown in P ig.l. We treat the problem etomk beam at the density matrix level 'flu and bring in the light-shift term s, which arise from the strong off-resonant field, into the equations of motion and solve these in the steady- state and obtain expressions for the fluorescence signals. % The geometry used for • f 10.1. IctaiMlte rflivran i I o «N r Ike ■rv# polarfsftttoiu of ihe rorlovo brums. studying the optical Hanle effect is shown in Fig.2. The atomic coherences are moni tored by observing the fluorescence signals in different directions. The fluorescence signals studied here are - the detected radiation in. the direction (i) x w ith p o la rization along the y direction (Ly), (ii) y with polari zation along the x direction (I*). The circularly polari- ■=> 103 = zed off-r6sonant light lifts the degeneracy of the exci ted states and also shifts the ground state. In effect, it acts like the magnetic field, the difference being that the magnetic field does not shift the ground state. When a two-level atom is subjected to an of f-resonant- field, its ground and excited levels shift by equal amounts but in opposite directions, the magnitude of this shift being proportional to 0*/&o where 0 is the Rabi frequency of the transition and is the detuning. A left-circularly polarized light shifts the levels and |g> by equal amounts in opposite directions. We write the interaction Hamiltonian for the atom and the two light fields in the electric dipole approximation and take into account the radiative decay and make a rota ting wave approximation to eliminate the fast time depen dence and use Bogoliubov-Mitropolsky1e method of time averaging to bring in the light-shift terms and finally obtain the Hamiltonian as H(t)= -(d+g.e(t)A +g* d_g.6(t)A _g+ H.C)+*(<00- Wl+»)A>+ + *(U0-U L)A__- 6 Agg, where 6= IG IV ^-C ^) and The steady s t a t e s o lu tio n s sore given by ( ^ ++^__)=2a2p[(4Yah2+4a9p+68) (62p+4YaH+4a*)+96a(6, -»-4Y,)li3tfa' which for 6~>0 goes over to 2aa/l4aa+Yan)• Here p = 1 + ( yc 7y ) and D»[Yapa+4aap+26a+ f f f i ^ T^ 3L(4YiVa+4aap+6a H 4Y V +6V +4aa ) +96a( 6 a+4Ya )p]+ (6 a/2)[9& a+(4YaPa+4aap+6a ) x [2aa(p-l)-p(4Ya+6a)}. The atomic coherences are given by Re’^r+_= s ap [ ( 6 a+4Yapa+4aap)(4YaH+4aa- d a )+ 9ial4 a+4Y, )M]D ^ 104 - which for 6-»0 goes over to aa/(4a3+Yan) and I 2a8p6y[96a(l-(i)-l4Yalia+4aaii+6aKl+|i)Jp”*' which goes over to zero aa 6->0. We further aee that L and Ly are aymmetric functions of 6 for e = 0, s/2 etc. end for the case when © » s/4, 3*/4 etc. the signals have the property that Lx(6)»Ly(-6) and vice versa. In the. lim it of a weak monochromatic pump i.e. Yo**0, a« Y » the results simplify to = « ' 1 6 - t , *" 4 d a+Ya ? * Ke^_ . («*/3) [ p*p + and 46*+ynr 3 lm % . = (2aa6/?Y) [ 6*^7* ” 4A 7 * 3 • Thus, each fluorescence signal can be w ritten as a sum of two Lorentzians. These results are in agreement with a recent experimental and theoretical work of Delsart et al’. The normalized plot of the signal L (6), i.e ., of L (&)/Ly(0), as a function of 6, for © = s/2, for the weak-fleld case is shown in Fig.3. This is a symmetric function of 6. It is seen that the line broadens ae the fluctuation parameter y0 is increased. It is to be noted here that the value 6=0 corresponds to the zero value of the intensity of the circularly polarized light. In F i g . 4 * the normalized plot (normalized so that the maximum value is unity) of the signal L y (6 ), f o r 6 = s/2, for the weak- field case is shown. The s i g n a l L y (o ), w h ich i s symmetric with respect to 6, has a double humped as- structure. In this case also, a similar effect, as seen in Fig.3 viz. , broa dening of the line with increase in Yc* obser v e d . H 0.1. The nnnlliS flaorieeoaoa sin * 1 I , ea o In Fig.5, a plot of facttoo of • for * - »/*. y l , «sd for the M ia * . b« rehire of the tundwkhO peroeirter y,i til T, ■ *1 the fluorescence signal M------y.*'• (cl--—- r,-»l Ml——T,-l*l M Lx(6) for d = s/4, for r.oiei (ft- r.-w. the strong field case is shown. For the narrow-band excitation (y =0), the line •It ; fW . 4. TW w n l l n d l > i w » i w ««>n* l t , >• > fMnMmef • ter tlw n m «d w e «* peremetare aa ta |r» ».. -3 0 -10 •It has a single peak with n o . $, The Ouareeeeaee alaaa) L , «» • hertien at» slight asymmetry , about fcr «* »/«. o»IO. v - 1. a»> far tataaa at lla'W aW M 6 aa 0 . AS the laser Z 1U C — parameter v# «nt la) T,*Ol S I------yt . 1 |r) tuations increase, the -r,-«i tfl------r, m asymmetry becomes pronoun- ~ ...... ced and the lin e ac q u ire s a d is p e rs iv e n atu re . However i it is to be noted that the signal always remains positive.. References : 1. See for example the articles by H.Walther and by B.Decomps, M.Dumont and M.Ducloy in " laser Spectroscopy" edited by H.Walther (Spr inger-Verlag, New York, 1976), Vol. 2. 2. V.B.Kaftandjian, 1.Klein, Hhys. Lett. ££ A,317(1977); V.JP.Kaftandjian, L.Klein and W.Hanle, ibid. jfi. A, 188 (1978). 3. G.Delsart, J.G.Keller and V.P.Kaftandjian. Opt. Commun. 52, 406 (1980). a a a a o - 106 - NONLINEAR TRANSIENT-AND STEADI-STATE RESPONSE IN A CIASSIGAL MODEL MarynaW'D1 Souza,• * Richard D*Souza, +' and Arvind Kumar * * Department of Physics, U niversity of Bombay, Bombay 400098 + Spectroscopy D ivision, B.A-.R.C., Bombay 4OOO85 I t i s a fam iliar fa c t th a t a (damped) harmonic o s c illa to r in interaction with electromagnetic field acquires, in steady state, a fixed phase relative to the field and this phase corresponds to absorption. In the transient regime, the oscillator can have an emitting phase; however, an ensemble of harmonic oscillators with random initial phases is again always absorptive even in the transient regime. By contrast, a quantum mechanical atom can ’adjust' its phase relative to the field to give, either emission or absorption. These features underlie the failure of Lorentzian theory and the success of semi-classical theory to account for stimulated emission, provides a model for laser action and describes coherent transients. A question of obvious academic interest is:, can a classical model based; on anharmonic o s c illa to rs simulate- the ty p ical nonlinear e ffe c ts of the quantum o sc illa to rs? Hlhe anharmonic o s c illa to r has been, of course, a recurrent theme of study in widely different contexts? Specifically, it has been considered for nonlinear optical 2 effects such as higher harmonic generation etc. As a possible model for laser action it has been investigated in great detail by Borenetain and lamb.^ In this work, we investigate the resonant transient and steady state Doppler broadened response of a thermal ensemble of anharmonic oscillators, with initial random phases with = 107 - ■ respect to the.-incident laser radiation. We first set up the dynamical equations of the theory, obtain analytically the steady state response up to third order in electric fie ld and.flM :;a fe atu re in sharp.contrast to the quantum case: the nonlinearity in classical response -vanishes at resonance and up this order the ensemble is essentially in a linear unsaturated absorbing phase. Away from resonance, the response depends, unlike the quantum case, on the sign of the detuning. The Doppler integrated Free Induction Decay signal is also seen to vanish up to th is order, under the usual approximation of the homogeneous width being sm aller than the Doppler width. We next consider the transient regime and prove analytically that the third order transient polarization is opposite in sign to the linear polarization - a feature important for strong signal phenomenon of nutation. However, the d etailed co efficien t of the nonlinear term does not suggest the simple nutation-like behaviour of a quantum o s c illa to r. The conclusion of the paper is: resonant nonlinear effects such as saturation and optical- nutation are more intrinsically quantum mechanical than laser action. Physical arguments are presented to support this conclusion. REFERENCES , , 1 Feld M.S. 1977 in Frontiers in la s e r Spectroscopy, edited by R. B allan, S. Haroche and S. Liberman (Worth Holland); Bogoliuboy N.N. and Mitropolsky YJU 1961 Asymptotic Methods In the theory of Wonlinear O scillations (Gordon & Breach, Hew York) 2 Maclean T.T. 1977, B.E5. - SMR- 2 0 /1 ; Harduccl L.M., Mitra S.S., Shatas R.A., Coulter C.A., 1977 Phys. - 108 - Rev. A 16, 247 3 Borenstein M. and Iamb ff.B., 1972 Phys. Rev. A 129® - 109 - PHOTON* ECHOES GENERATED BY THREE EXCITATION PULSES HARI PRAKASH and GYAN SWAROOP 3HATM AGAR Department of Physics, University of Allahabad, Allahabad, U.P. 211002 When a pulse of radiation, falls on. a collection of resonant atoms, a macroscopic dipole moment is created, which decays because of the homogeneous and inhomogene- ous relaxation processes. If a second pulse of resonant radiation, falls after some time interval, the process of dephasing of the atomic dipole moments due to inhomoge- neous broadening, is inverted and a ’rephasing' starts. After., a lapse of another equal time interval, this 'r e p h a s i n g 1 leads to build up of. a macroscopic dipole moment and it results in spontaneous emission of a pulse of radiation which is called photon echo. Amplitude of the echo depends on the homogeneous transverse relaxa tion.tim e and its width, on. the inhomogenous relaxation time. One can determine the transverse homogeneous relaxation, time from the measurement, of echo intensity for. various values of. the intervals between, pulses. Phase-matching requires collinearifcy of the two excitation pulses and the echo pulse and produces d iffi culty. in detection. Echo pulse has been separated by using (i) optical shutters, (il) detectors with small response time and large intervals between, pulses, (iii) “ 110 - an- axial magnetic field which, rotates the echo polariza tion:, or by Civ) taking, non-c.ollinear pulses and sacri ficing phase matching.; Recently, backward stimulated photon echoes genera ted by three excitation pulses have'been reported for both solids and gases. These echoes can.be. easily sepa rated by adjusting the polarizations of excitation pulses in such a way that-the polarization of the echo, pulse is perpendicular-to that of'oppositely propagating excita tion pulse and by using a glan prism, for gases, however, one has to sacrifice slightly the perfectness in phase matching to make the experimental detection feasible. The stimulated echo amplitude depends on the longitudinal homogeneous relaxation time and, therefore, measurement of the intensity of this echo can give the value of the longitudinal homogeneous relaxation time. We have studied in detail the three pulse photon echoes for both solids and gases. We get six terms in polarization, corresponding t.o echoes but , at the most., only four echoes can be obtained after.the third excita tion pulse. Of these four, one echo is automatically separated from the excitat-ion pulses without using any special device because phase.matching demands that, the A A A A unit, vector along/ its direction is n = 2n^ - 2n 2 + u -|, where n^ (i = 1 , 2 , 3 ) is unit-vector along the direc- - tion of the ith. pulse. Perfect, phase matching is also easily achieved for this echo. This can be used for - determining the transverse homogeneous relaxation time. We also show that the loss o f intensity of; the backward stimulated echo in gases due to the slight sacrifice in perfectness in phase matching required for experimental detect-ion is large and depends on intervals between the excitation pulses. An explicit accounting of" this loss is thus essential, if the backward stimulated echoes are used for determination of relaxation time. The other phase matched echoes are: obtained when A A A A A A A A A A n = 2n2 - n 1 , 2n-j - n 1 , 2 n j - n2 and n^ ± (n2 - n1 ). Amplitude of the echo corresponding to n = 2n-j' - n2 depends on. both the transverse and longitudinal relaxa tion times and intensity measurement of this can lead to determination of both relaxation times. If we keep n2 = Q3 / n.| 1 phase matching conditions give only the echo correponding" to the relation n = 2n^ — n 2 along n2, and the echoes corresponding to n = n^ - n 2 + n^ and n = 2n^ - 2n 2 + n 1 along n1, and the former echo is thus separa ted from the other two. The amplitudes of the echoes corresponding to n = 2n 2 - n 1 and n = 2n^ - n 1 dep en d on the transverse homogeneous relaxation time and the measurement of intensities of these echoes can thus give the transverse homogeneous relaxation time. - 112 - LASER-INDUCED RAB-I FREQUENCY QUANTUM BEATS ■ IN SUPERFLUORESCENCE RANJANA PRAKASH and NARESH CHANDRA Department of..Physics, University of Allahabad, Allahabad, tf.P. 211002 All previous studies on quantum, beats fa ll broadly into two groups, viz., (i) quantum beats when the atomic transitions producing, quantum beats have a common term i nal level1’2, and.(ii) quantum, beats in superfluorescence from an assembly of two. different types of atoms^’^ in. which the two transitions do not. have a common term inal level and the coherence, necessary, for- observation of beats is generated by interaction with a common radiation field^. In this paper, we study superfluorescence from an assembly of identical, atoms involving a transition be between two singlet,states in presence of an intense laser beam which resonates with the lower of the singlet states mentioned and some.other atomic, state. We shall show that these, laser-induced quantum beats are due to the Rabi phenomenon and the beat frequency is equal to the Rabi frequency which is modified in this case due to strong interaction with a third atomic level. We consider, the assembly of atoms contained in. a volume whose dimensions are much., sm aller than, the "mean. > wavelength of superfluorescence and the laser, wavelength, - 113 '"and the laser beam having, one mode and euch a high inten sity that it:.can be treated classically. We find that, in the special case of a sharply, toned intense laser beam, the beat, frequency is equal to the unmodified Rabi fre quency, the phase of the. beatings corresponds to destruc tive interference at,.t. = 0, and., the fractional beat amplitude is 1 . We suggest-several experiments for observation of 5 this phenomenon. Gibbs, Vrehen and Hikspoors observed superfluorescence from a pure tw o-level system, by pumping cesium atoms which were initially in the state 63^2 (mj = —1/2, mj- = - 5/2) to the state TPjyg (mJ = ~ 3/2, nij = - 5/2), and then observing... superfluorescence to the state 7 3^2 (m = - 1/2, m = - 5/2). The duration, of super- fluorescence. is /« 10 ns. If one repeats this experiment in presence of a laser beam which resonates with, e.g.., states 75^2 (mj = - i/2, m-j = - 5/2) and 7Pr/2 (mj = - l/2, nij = -5/2) and keeps the laser intensity above the value corresponding to the Rabi oscillation time period #v 1 ns or. less as calculated on.the basis of matrix elements given-by Heaven^, the laser.-induced Rabi-fre.quen- cy beats in superfluorescenee can be observed. For this, laser. intensity of. the order, of. 2.9 W/cm2 at the wave length 3.096 |tm is needed. Dewwy, and Hooker? have obtain ed laser, peak, powers ~ several, kw in the region 5,-4 pm. - 114 - References. 1. R.L. Shoemaker- and R„G. Brewer, Phys. Rev. Lett. 28, 1430 (1972); S. Haroche, J.A. Paisner. and A.L. Schaw- low, Phys. Rev. Lett;. 30, 94-8 (1973); P. Schenck, R.C. riilboru. and.. E. m etcalf, Rhys. Rev. Lett. 3_1_, 189 (1 9 7 3 ). 2. R.L. Brewer and o.L.. Hahn, Phys. Rev. A 8, 469(1973), 1, 1479(E) (1974), U , 1641 (1974); I.E. aenitzky, Phys. Rev. nett. 33, 1755 (1975); G. S. ;Agarwal, Phys. Rev:. A 1_5, 2380 (1977); see also other papers referred to in the last reference. 3. Q.H.F.- Vrehen, M.M.J. Hikspoors and H.M. Gibbs, Phys. Rev. Lett.- 38, 764 (1977). 4. R. Prakash and N. Chandra, Phys. Rev. Lett. 4.2, 443 (1 9 7 9 ). 5. Gibbs, Vrehen and Hikspoors conclude on the basis of the transition dipole moments, dyg_yp = 1 2 , d g g_yp = 0.5, dyp_^p = 1.75 and dyg_gp = 4.4 in units of 10-1S esu, that the competing and cascading transitions are small. This ia also evident from the branching ratio 0.49 for the transition 7P3 /2 -* 7S1//’2> and from the lifetim e 57 ns of the state 73^2 which is large as compared to the duration, of superfluorescence, 10 ns. 6. 0 .S. Heaven, J . Opti Soc. Am. 5j» 1°58 ( 19 61 ) . . 7. C.F. Dewey, Jr., and L. 0. Hooker., App'l. Phys. Lett. 18, 58 (1971). - n 5 - * . STEADY PROPAGATION OF A SINGLE PULSE IN A TWO-PHOTOIv RESONANT MEDIUM. SECOND HARMONIC RESONANCE Hari Prakash and Gyan Swaroop Bhatnagar Department of iPhysics, University of Allahabad Allahabad-211002, India. Steady propagation of pulses of radiation in a medium having a second harmonic resonance is studied for pulse durations much less than the atomic relaxation times. This phenomena was earlier studied by Belcnov and Poluektov (1969). Takatsujii (1975) studied"the sim ilar problem involving summation frequency resonance. These authoe simplified the problems by leaving conductivity loss and non-resonant contribution to polarisation out of consideration. We consider second harmonic resonance and include into consideration a finite conductivity loss and also the non resonant part of this polarization. We use slowly varying envelope approximation to solve the prob lem completely, and find expressions for the pulse shape, phase velocity, the envelope velocity, in phase and in quadrature components of the atomic dipole-moment, and the population inversion. We get exactly resonant non-cliirped single pulse solutions. Narducci, Eidson, Furcinitti arid Eteson (1977) made some comments on the problem of steady pulse propagation in a two photon resonant medium while giving a theory of two photon laser amplifier. These authors have concluded that (i) steady pulse propagation is possible only when the loss is less than a threshold value (ii) multiple steady state solution are possible. We show that our results support the conclusion (i) but contradict conclusion (ii). For a lossless medium a Lorentzian variation of intensity is obtained. If the loss is dot negligible but still quite small it is seen that the amplitude is given by hyperbolic secant to a very good approximation. For lossy medium, in general, a numerical integration is needed. For loss greater than a threshold value no steady pulse solutions are obtained. References : 1. E.M. Belenov and I.A. Poluektov, Sovt. Phys., JETP 2£, 754 (1969). 2. M. Takatsuji, Phys. Rev. All, 619 (1975). 3. L.M. Narducci and W.W. Eidson et a l., Phys. Rev. Al6, 1665 (1977). ty t EXTENDED ABSTRACT NOT IN PROPER FORMAT - 116 - COHEHMT OSCILLA.TIONS OP ATOMIC BEAM # Jahangir B. M istry and Richard D'eouza Spectroscopy Division, B.A.R.C., Bombay 400085 Consider aa atomic system, having transition frequency (•^between the states |a> and |b> (the upper and lower states respectively), Cook^ has shown that the net rate of momentum transfer to the atom, i.e . the radiation f o r c e i s I - BW(Pb*Pa)tk (1) where B is the Einstein B-coeffloient, V is the energy density of the wave, Pft and P^ are the probabilities, respectively, that the upper and lower resonant levels are occupied and k is the wave vector of the laser beam. If Pt > Pa, the system undergoes a push in the direction of the laser beam. If P& > P^ i 0e« for a system which has a population inversion the system: recoils in a direction opposite to the laser beam due to "anomalous" radiation force.. Note that in addition to the radiation force, there is a classical momentum push, which, however is negligible. Spontaneous emission has also to be considered but the recoil is isotropic and its contribu tion can be made negligible. If the system is in the form of an atomic beam and is irradiated perpendicularly (to eliminate Doppler broadening) with a resonant laser be a®, these effects can be observed &e end deflections of 2 •• L th e atomic beam. The strong - signal traraitlon probabilities Pa and P^ 5 are given by ■ 1 ~ Ptta y . ( ^ - V )2 * E / i )2 w here ( f Is the magnitude of the eleetric-dipole m trix element and B is the amplitude of the laser electric field. (Note, at t=0 Pa=9 and P^=1 ie the system is in the lower state). Equations(2) shows that the population oscillates with frequency j* (Rabi flopping frequency). Prom equations (1) ard (2) we see that the end displace ment of the atomic beam should oscillate with frequency p.. This however, to the best of our knowledge, has not been observed experimentally. The reasons could be that all such experiments have been done either in the spatial or frequency domain and not in the time domain. The end deflection!of the atomic beam have been measured with respect to distance perpendicular to the atomic beam. Other experiimntors have used fluorescence or another laser beam to monitor the frequency response of the system as the strong laser is tuned in frequency. If the - 118 experiments were done at fixed frequency (resonance) and ''a£?a given point in space (eg. atomic beam and laser interaction region) the population oscillation - as the system absorbs and coherently re-emmits the resonance radiation - would have been observed. ACKNOWLEDGMEJTS 1 We acknowledge the constant encouragement of Dr. N.A. Warasimhsm and Dr. M.N.Dixit (Spectroscopy Division, B.A.R.C.) during the course of our work. We thank Dr.K 0. Rustagi (laser Section, B.A.R.C.) and Dr. A. Kumar (Sombey University) for valuable discussions. * Permanent address: Physics Department, St. Xavier’s College, Bombay 400001 REFERENCES 1. B.J. Cook, Optics Commun. 22(1980) 235 2. R.Schieder, H.Walther and l.W oste, Optics Commun. 2(1972) 337 3. J.-L.Picque and J .-l Vialle, Optics Commun ,5(1972)402 4. E.Arimondo, H.lew and I.Oka, Laser Spectroscopy IV eds. H.Walther and K.W.Rothe (Springer-Veriag 1979) 5. M.Sargent, M.O.Scully and W.E.Lamb, Laser Physics (Addison-Wesley, Reading, M ass., 1974) NON 1XSEAB OPTICS Theory of laser beam instabilities - N.C.Kothari and S.C.Abbi. Two photon absorption studies in Ods single crystal — A.S .Inamdar, H.P.Borgaonkar, N.Mehendale. An accurate TFE determination of material media - K.K.Sharma, H.L.Goyal and S.P.Srlvastava. Doppler free spectroscopy in modulated laser beams - B.Saxena and S.S.Agarwal Second Harmonic Generation In a Free Electron Laser - Earn Babu and H.Prakash Theory of second harmonic generation in a "stack of plates" - S.Meenakshi and K.C.Buatagi. Theory of optical 3 wave mixing in a stack of crystals — S.C .Mehendale and K.C.Buatagi. Theory of second harmonic generation in rotationally twinned crystals - K.C.Buatagi and 3.Meenakshi. Harmonic degradation' of serveral optically birefringent crystals in the near and far fields - K.K.Sharma, M.L.Goyal and j.P.Srivastava. Temperature tunable coherent infrared generation in nonlinear crystals - 3.C,Ghosh, P.S.Ghosh and G.C.Bhar. - 119 - THEORY OF LASER BEAM INSTABILITIES N itin C. Kothari and Satistv C. Abbi Physics Department Indian Institute of Technology Hauz Khas, New Delhi - 110016 Instability growth plays a central role in deter- , mining the fate of an intense laser beam propagating in a Non-linear media. Instabilities are known to grow exponen tia lly in non-linear media and are also known to produced stabilized filaments as a result of their growth. These seemingly contradictory effects were not understood in any e x i s t i n g theories^of the phenomenon till the concept of 'Local Energy Conservation Principle1 was incorporated in k ;the theory of Filament formation. The new theory consi dered uniform intensity background laser field /£„/ and a complex field perturbation ( e , + i e z ) given by e /,z - etc>2c e (^Stn(kj.x.x)S^c^lj) when b 'j.ij relates to the character istic size of the perturnation and o C (2 ) is the growth parameter. By invoking a j Local energy Conservation' criterion which required the energy in cells of size X. T f j to be conserved , the theory was able to explain (a), that, for specific range of characteristic sizes, the energy in the perturbation stablizes to a constant value at a well-defined distance inside the non linear medium, (b) that for a specific characteristic sizes of the perturbation the distance at which the stabi lization takes place is the shortest.and this distance 120 - compares well with the experimental distance at which "the beam breaks up (c) that for stabilized experimentally observable filaments ih is size is nearly constant. The above theory gave good qualitative agreement with experimental results but a quantitative agreement can be obtained if we consider a more realistic two quassian model for instability growth^. In thi-s model the laser beam profile is taken to be gaussian viz: 2 2 E (*•) = e and the complex perturbation f 6, 4 v€z) I s a ls o taken as a guassian viz: <*£*) _ / t / 2 b % ) 2 " ^-/Oz2o ^ . € A two step energy conservation criterion applied td, this case, gives us the. distance for the appearance of filaments as: t/fj w here / ^ ic - f C2 c measures the initial*size-of the perturbation and N is given by L & (E jZ£Sf . y t t f v In this approximation we obtain a clear distinction between Z>y referred to as the 'Self-focal distance' in literature and X t j the distance for the first appea rance of filaments or beam break-up point. - 121 - A more exact treatment of the two gaussian model requires the use of a continuous overall energy conserva tion criterion. This treatment gives the precise value ifor the distance into the non-linear medium at which team break-up takes place. The result is where /\j> is the wavelength of la'ser light in vacuum and A and B are given by I n & - ' — A w here !fj o is the optimum value of ’T j obtained from equation _/ r _ 2. . a M - 0 ' r ' j 3 . C 2. cjU / - 7 • '<■/ The evaluation of experimental parameters like the ‘distance for the first appearance of filaments , the ;size'of filaments end their brightness can be ocfen-et.-d from this theory with aid of a computer. It is also possible to include the effect of linear [and non-linear absorption in the theory of filament formation. The results of a series of computer calcula tions indicate some interesting possibilities regarding - 122 - Laser Fusion type of: experiments. Controlled generation of giant filaments seems to be a promising way of producing high energy densities needed in Laser Fhsion. To design such an experiment would reguire considerable computational work as well as a series of preliminary experiments to determine some basic parameters of the plasma. The proper ties of the plasma should be incorporated in the theory in determining the exact, characteristics of the laser beam as well as a well designed perturbation so as to allow an exact balance of laser propagation and plasma instabilities. A monumental amount of work needs to- be done but it seems plausible now that a successful.Laser Fusion experiment may be a well designed experiment using Lasers of moderate powers rather than the brute force methods using very high power beams from many lasers simultaneously. R e fe re n c e s 1 . J.H. Marburger, Progress in Quantum Electronics 4, 35 (1975). 2 . Y .R. Shen, Prog, in Quantum Electronics' 4,1 ,(1975) 3. O.Svetto, Prog, in Optics, 12 (1974) 4 . S.C. Abbi and IQ.tin C. Kotharl, Phys, Review. Letters, 43, 1929(1979). 5« S.C. Abbi and Nitin C. Kotharl, Journal of Applied Physics, 51, 1385 (1980) 123 - Two Photon Absorption Studies In CdS Single Crystal A. S.Inamdar, H.P.Borgaonkar, Neela Mehendale Department of Physics, University of Poona, Pune 411007, TPA studies in CdS Single Crystal are reported by many investigators (1-3) using either Ruby Laser, Nd laser or He-fie laser. The present work deals with the TPA studies in CdS Single Crystal using NLPD laser. The prystal used for present work was a pure CdS Single prystal, obtained from NCL Pune. N2- DYE LASER LASER. OSC- + AMP X “ t R.ECOR.TE.R. Figure 1 shows the experimental set up used. Laser be am was focused on CdS Single Crystal along its C—axis by means of a lens. The emitted fluorescence - waa collected at right angles through a green filter. A prisminono chroma tor, EMI 9804q b photomultiplier, Box—Car averager and X—t recorder formed the detecting s y s te m . Fluorescence signal was recorded for different incident laser wavelengths; between 5 6 0 n’m to 640 nm obtained from Rh.6G.and RhB dyes. Collected signal when scanned gave the laser wavelength, relative laser intensity along with the emitted fluorescence signal a t 488 nm. ^ 1^ ) q E - 125 - 3.9 eV to 4.3 eV. References I 1 . F . Shiga and S.Iinamurai Phya. Lett. 25A. 706 (1967) 2. S.Carusotto, G.Fomaca and E.Placco; P h y .R e v . 157. 1207 (1967) 3 . F.Cornelti , G.Fomaca and M.Vaselll; J .Luminesc, 8 ,- 462 (1974) 4. M.B.Brovn et al. J.Lumlnesc, 1, 78 (1970) 5 . M.K.Sheinkman et al. Sov. Phys. Solid State 10, 2 0 6 9 (1969)• An Accurate IPS retermina"Lion of M aterial Media K.K. Sharma, M.L. Goyal and G.P. Srivastava Department of Physics, University of Delhi -110007• With the advent of ruby laser, Kaiser and Garett (l)first experimentally observed TPA at optical frequencies previously predicated theoretically by Gopper - Mayer (2) i n 1931» Several techniques have subsequently been used to find TPA ■ coefficents both, theoretically and experimentally for example by measuring the intensity dependent trarismission through two photon excitation.(3) or by analysing the TPF spectra (4). Several comprehansive reviews of TPA are now available. Owing to v ario u s advantages o f two photon flu o rescen c e spectroscopy in furnishing the details about the structure and)' or nature of the molecules, of a media an accurate and simple method has to be adopted in studing the fluorescence charact- eristics of the samples. Absolute TPA coefficients can only be obtained directly by well calliberatea lasers and utilizing highly sensitive detec tion unit within the region of interest. However, relative measurements can be carried out quite simply by the two channel detection technique (5). Utilizing known Baman cross-sections, this technique then yeilds absolute TPA coeff. of these mater ials. Parametric mixing experiments such as three wave mixing have also been utilized by several authors (6) to. measure the real and imaginary parts of the 3rd order non-linear suscepti bility and to determine their TPA cross-sections. Experimentally substantial disagreement between the publi shed values of (transition rate) or£as shown by the large scattering of the results of this quantity have been indicated by several authors. This state of affairs is unfortunate. A slightly modified and more accurate experimental method is now being utilized by the authors for measuring the TPA fluorescing characteristics of these componnels. The method may reduce several experimental errors and resolve certain experimental lim itation in TPP measurements. This work is in,.e^tehtlon:of.,i the sim ilar type of work on EPF measurements.by_2ucunning and o th e rs ( Y ) on several fluorescent materials utilizing two uncorrelated detectors for. detection purposes. A single detector is now used that may avoid any correlation beween the detection/displaying units and fasilitates observations with equal accuracy simultaneously. . Theory Behind The Measurements And Accuracy : Firstly, the sample (if in the solid form is dissolved in- a suitable solvent to a known concentration ie. anthracene in cyclohexane; coumarin in ethanol, etc) is taken in a rectangu lar cell and emulsified with water so as to form an undissolved water matrix throughout the concentration of the cell. The method may apply to several organic materials and related compounds which remain unreacted with water as an emulsifing medium. The method adopted is the same as that applied by the previous authors (9 ) for analysing the Raman scattering cross- section of several, organic solids. Tne demension of the cell in the present study may range from OyOl cm^ (1 x.1 x.1) to 1 cm^ (1 x 1 x 1) depending upon the quantum eifieiency/concv of the solute. The excellent thermo-optical properties of water matrix help to prevent refractive index gradients from degrad ing TP fluorescence spectra and neccesiates accurate measure ments. This may prove t'o be important specially in MPA studies, when the sample degrades under high energy excitations. Secondly, the applications of the. most direct method (ie. measurements involving the absorption induced in a weak probing beam a t w0j by an in te n se beam a t w^a wq ) and the non too acc urate indirect method (i.e. fluorescence quantum yield measurements] l/l7CfodUiC40 /cefctain experimental problem in the measurements of TP fluores cence parameters. A few main difficulties associated with these measurements are: A precise knowledge of the following fectors are involved (1) Fraction of the emitted light collected AF (2) Transmission characteristics of the collecting and filtering system. (5) The photom ultiplier gain - 128 (4) 1'be photocathode quantum- efficiency of PMT. The overall effect of these quantities on the TPA cross- section can-be easily understood from the following considera tions. The experimental- arrangment to-relate one photon to two photon processes samultaneously has been shown in sa Fig.U he photocurrents. . detected (solving as a function of radial distance r )b y the two PMT’S I an'd’"ll are given by : jr„p (T> = c'HoX* O0r)J V tr,tl d r cohere p» = r ^ Q) = S /or *=2- C* is the colliberation constant of the detector which may very for the different photomultipliers used for the measurement of light photon flaxes Ffi per unit area; and b(T) = ^ J C5‘) - 0 is for the rise time of the response of the detectors. Assuming the two detectors to be calliberated separately for the conci- dent laser flttxes and Fg» the two photon cross-section is then given by the relation i (3) £ = Where Q i s th e n o n -lin e a re ly o f th e medium producing double frequency (SHG) for one photon measurements. The S'-j and SJ, represnt the irradiated surface area for one and two photon excitation, respectively which may not in general be equal under orderafealy experimental conditions taxing S1(r) = S' and again f = f dV ) c- S , *J S i. may introduce significant amount of errors in TP fluorescence v n measurem$s, provided the values of S1 and F^ are precisely known from accurate experemental measurmmiits. The introduction of coloured filters and Cg (figure 1) in the detectionunit in order to absorb thefundamental may also influence the accuracy of the system. This is due to the fact that C, and C., A a (figure 1) in the detection unit in order to absorb the fundamental - 129 - may also influence the accuracy of the system. This is due to the fact that and. Cg may not be transmitting equally v.el i. throughtout the region of interest. Figure 2 shows the exper imental arrangement.— ' ■ — where the sample has now been placed preferably in a light tight box and analysed for TPE by the d e te c to r c o n s is tin g o f 2~57 f i l t e r s ( 7 0 0 0 A0) and coloured filters already caliberated for IDTPE (Intensity dependent two photon excitation) for the samples. The variation of IDTPE with incident laser flux F for the samples has been shown in fig. .3. This method brings in a calliberation constant k for the detection. For an ititial laser pulse (duration T sec), the value o f £ is then readily obtainable from the relation ; J A , f 2 where t ^ and 1^ are now very accurately determined depending upon the accuracy of the detector. Besides this the depend ence on statistical nature of the beam can also be well established from the accurate measurements on 1^ simultaneously. The authors are now using it for the determination of MPA fluorescing characteristics of several organic and related laser materials in the laboratory. References: (1) W. Kaiser and G.C.B. Garrett, Phys. Rev. Letters £ 229 (1961). (2)- Goeppert M. Mayer Annder Phys. 9 (1931) 273. (3) S. Singh and J.E. Geusic, Phys.' Rev. Letters 17.865 (1966). (4) E. Panizsa and P.J, Regensburger, Phys. Letters 24A 321 (1967). ' (5) P.D. Maker, R.W. Terhune and C.M. Savage. Physical Rev. L e tte r V o l.12, Wo.18, 1964. (6) L.P. Bloernbergen N. Phys. Rev. B. 17 (no.12) 1978. (7) J.P. Hermann and J . Ducing Phys. Rev. A Vol 5i Wo. 6 * (8) I.B. Berlman, Handbook of fluorescence spectra 1972 of Aeromatic molecules (Academic, Wew York, 1965). (9) K .L . Matheson, J.M. Thome, Appl Phys. L e tt 33 (9) 1978 - 130 - Wo 2-5 V - 131 - ' DOPPLER PRHS SrBCTROSCOPY IN MODUi a Th C LASSE. jjEa KS R. Saxena-. and G. 3. Agarwal School of Physics, University of Hyderabad, Hyderabad The resolution of spectral lines of gases in the optical range has long been limited by the Doppler effect which gives each line a relative width of a few GHz. Recently, several techniques have been proposed and used to improve the linewidth in this respect, some of the well known ones being atomic and molecular beam spectroscopy, two photon spectroscopy and saturated absorption- Spectroscopy^^. To further increase the sensitivity of resolution, Sorem and dchawlow^^employ ed saturation by two oppositely directed waves which are intensity-modulated at different frequencies. The narrow resonance of ezcited atoms is detected by re cording the fluorescence intensity oscillating, at the sum of modulation frequencies, a high resolution study of the hyper fine structure of molecular ioair.e was obtained by.using this technique. This method has a decided advantage when working with very weak transi tions, with very low pressures (where the total absorp tion of the atoms or molecules is small) or with a sparsely populated lower state. In view of the practi cal importance of this method, we undertook a detailed • theoretical study of this hitherto neglected technique. Using perturbation techniques on the Bloch equations, intensity of the fluorescent light oscillating at the sum of the modulation frequencies was obtained for a general relaxation theory. This being particularly important since the experiment was performed with the molecules in a vapor cell. Analytical expressions were obtained in the Doppler lim it, these reducing to the results of Sorem and Schawlow for the case of small modulation frequencies and radiative decay only, we wish to extend"the study to the case when one of the travelling- waves is intense, we expect the narrow resonance to exhibit the dynamical Stark splitting free from Doppler broadening. The electric field of the two oppositely directed laser beams whose amplitudes(hence intensityj are time modulated may be written as + S1(t) = C10 (l+a^sin ------) exp[-i(tOj-KL.V )tj+c.c. B2 ( t ) = ? 20 (l+a2sin’^ |— ) exp[-i( (D^+KL."?)t)+c .c. Using perturbative methods on the well known Bloch - equations to fourth order in .a(t), where oC ( t )= a1 ( l+ a ^ s i n ”r p ^ ) e 1 * ! ’ ™ + agU+agSin'^Je-^ !'^ we obtain the following expression for the intensity of modulated fluorescence (this being proportional to <(SZ{°°))>(4 )» where <(sz(00 represents the population difference at steady state) V L6a- ^ 4- 2 ------WJ[ t *i( S.+^>> (■^2 . = ^-66 Hz,= 853 2z in Sorem and Schawlow's experiment) with purely radiative decay, we finally obtain the simple result /a a(00 )\ ,. . -Is ala2al°2 c»s(J\+ - 0 2 n \ YU) 4 h - T — — ------ Thus if we detect the fluorescence signal with a photo multiplier tube and subsequently separate the required component with a phase sensitive detector referenced to t we obtain a narrow line at zero laser d etu n in g Whose width is limited only by the natural width I The unmodulated or d.c. component of the fluo rescence under, the same conditions is just a Doppler broadened line w ith no such s tr u c tu r e . ■•DC component ( Ji. +-Q_ ) component We hope to complete the study for the case of one travelling wave being intense, so that the narrow structure will then exhibit dynamical S^ark split ting free from Doppler broadening. R eference [l] H.W'alther in " Topics in Applied Physics" , (Springer-Verlag, Hew York, 1976), Vol.2 Chap. 1 and the references therein. [2J M.S.Sorem and A.L.Schawlow, Qpt.Comm.5, 148 (1972). aaaaa - 134 - Second harmonic Generation In a' Kraa Blecton Laser. Bern 3abu* and Kiari Prakash Pay s i c e Depp t . , Allahanad Universicy Allahabad - 2 Heoantly, a New type of source of coherent intense l i g h t u sin g stim u lated ei.dasion of bre,ms3trothlung in a spatially periodie static magnetic field known aa free- . electro a laaer naa been developed. In tne experimental set up an accelerated electron oeam ia made to paaa in a region having a periodic spatial variation of magnetic field in a oavity formed by two mirrors. In this paper it is pointed out tnat in frea-electron Laaer higher frequencies woich are very nearly equal to the harmonica are also generated in appreciable amounts. Second har monic generation in stimulated emission of bremgstrahlung ip a spatially periodic magnetic field ia studied using the Peyoman diagram teohniqes. '^he r a tio of the gain f o r the second harmonlq generation to that for tue generation to . that for the f&ndamantal ia of the order of 10-9 for the conditions present in Slias, Schwettmau and Smith, ‘The ratio of the intensity of the Second harmonic to that of the fundamental is also of the order of 10 ^ . Phe content of higher harmonica can also oe found similarly but these are expected to be very arr.ll. *Ham -ha eu. CMD Post graduate College bi la spur (M.P.) # EXTENDED ABSTRACT NOT RECIEVED - 135 Theory of Second Harmonic generation In a "Stack of Plates" 3 .Meenakshi and K.C.Bustagi Laser Section Bhabha Atomic Be search Centre Bombay-400085 Bor efficient nonlinear optical frequency conversion one has to compensate for the phase mismatch caused by ..dispersion between the generated electric field E(2a>) and the source polarization . The usual method is to exploit the birefringence in optically aniso- ( 1) tropic crystals. An alternative method is to use a stack of crystals oriented so that the sign of nonlinear coupling is opposite in adjacent crystals. This introduces an additional phase of 71 between and E and would compensate the phase change caused by dispersion if the lengths of each plate are chosen suitably. If the pump depletion is neglected, the length of each plate is 7t /jAK^ and ( 2 ) the stack of plates behaves like a hypothetical phase matched non linear crystal with its nonlinear coupling coefficient d@^ reduced by 2/j[ . In the present work we present a generalization of this theory to include pump depletion. We note that using a stack of 19 ( 3) GaAs crystals Thomson et al have already reported SHG of COg laser with 6% intensity conversion efficiency. The theory is best described in terms of normalized electric- field amplitudes u and v of the fundamental and second harmonic waves: u2 + v2 =1. The crystal length £ is scaled to^^ _ 4 7T CiP d eff. j ATfcoW c l k.Coj2-* , V d k i cc%‘-^ x where W is the total power incident and def^ is the effective nonlinear susceptibility. The other important parameter is S s A K l / g , the normalized phase mismatch due to dispersion. We use the coupled- wave solutions of ABDP to evaluate the second harmonic amplitude in each crystal with the output of previous crystal serving as initial conditions. The crystal lengths are so chosen that the relative phase G .= AK< +4>2 - 2 changes by 71 on passage through each crystal. Unlike in the undepleted pump case the length of successive crystal increases from =* ( 7T /|A KO (1 -(2/&S)2)in the first stage to a limiting.values (7T/^,K0((lASl-h I / | AS) ) as full pump depletion.- - 136 » is approached. Both expressions above are valid to order 1/( AS )^. Ihis dependence of the crystal length is one manifestation of the fact that nonlinear coupling not only changes the relative amplitudes'; of the two waves but also their relative phases. Another manifesta tion is that starting from suitable initial conditions full conver- ■ sion to the second harmonic would be possible in an infinite crystal even if A If ^ 0 but A 3 < 2. For large AS( > 10) the growth of the second harmonic is very similar to that in an equivalent phase matched, ciystal with dgff reduced by 2/ x • To conclude, we note that for high power SHG in the infrared stacks of cubic zincblende crystals such as GaAs and CdTe appear very promising. Among others, two advantages are the absence of o b ta in in g , beam w alk-off problems and the possibility of Avery nigh power handl- ing capability by flowing cooled inert gas through the stack. References; ■ 1= J.A.Armstrong, H.Bloembergen, J.Ducuing and P.S.Pershan Phys. Rev. J22, 1918 (1962) (ABDP) 2. J.D.McMullen J.Appl.Phys. 46. 3076 (1975) 3> D.E.Thomson, J.D.McMullen, D.B.Andersen, Appl.Phys.Lett. 29, 113 (1976) - 137 - Theory Of Optical 5-wave nixing Ia A Stack of Crystals S.C.Mehendale and K.C .Rustagi Laser Section Bhabha Atomic Besearth. Centre Bombay-400085 We consider the nonlinear propagation of two coupled mono chromatic waves in a stack of crystals whose orientations are chosen ( 2 ) so that % changes sign in successive crystal segments. We normalize the three electric field amplitudes V., V_ and V, of the 2 2 2 three waves , u)x and = CO, + COt such that + Vg + = 1 (Our amplitudes are. related to those of A3DP by V, = LLi • .---=__ „ _ 51) ? = / to, / the ratio of frequencies. It is interesting to note that for 3 wave mixing in a stack of given equal crystal lengths, the frequency selectivity would be high only for large A S. For smaller AS i.e. at higher intensities many processes involving generated waves could become efficient simultaneously. Then by suitable design of the stack, it should be possible to obtain DFG efficiencies higher than the Manely Rowe limit References: 1. J .A. Armstrong, H. Bloembergen, J. Ducuing and P.S. Pershan, Physical Rev. 127. 1918(1962). 1 Z 3 o 2. 4 6 a FIG. 1 O o a o e o ° o o 0 o U o ° '6 1 h > a ° 1------1------1------«- & to t s z o NUMBER OF CRYSTAL FIG. 2 - 139 - Theory of Second Harmonic Generation In R otationally Twinned C rystals K.C.Rustagi and S .Meenakshi Laser Section Bhabha Atomic Research Centre Bombay-400065 In this paper, we consider the second harmonic generation SBC (2) in a stack of cubic crystals such that dg^ changes sign in succes sive crystals but the lengths of crystals are given a priori i.e. they are not adjustable. If a crystal with cubic zinc-blende struc ture is rotationally twinned about the (111) axis, its nonlinear 'optical properties would be well represented by such a stack with lengths of each c ry sta l segment varying randomly^1 Dewey^1^ and coworkers have experimentally demonstrated enhanced nonlinear mixing in rotationally twinned ZnSe, Znle and CdTe. Since the length of each crystal is now given, the coupled wave solutions determine the relative phase 8 at th e .end of each stage. To determine this a computation of the normalized amplitudes (2) u,v as well as dv/dg becomes essential. This involves the compu tation of the Jacobi elliptic functions' which were calculated to an accuracy of 10~•6 using the arithmetic-geometric mean method (3 ) First, we consider a stack in which all the segmetits are of equal length. From arguments viewing fhasematching condition as conservation of crystal momentum one expects that this stack would behave lik e a phase matched c ry sta l i f the length of each segment 1 = IT / 1 A K t. However, due to the m odification of the relativ e phase 8 by the nonlinear coupling, we find that even for t = j f j |Aki» full conversion does not take place. As AS becomes very large, the phase modification by the nonlinear coupling becomes less important and correspondingly maximum conversion increases. However, for small AS, larger deviations in 1 from the optimum value are acceptable for substantial conversion. Hext, we consider the stack of crystals in which the lengths of oriented crystal segments are scattered at random about 7T/ i Ak| . In Fig.1 we show the growth of the second harmonic for | ASI = 10 for mean normalized length f = T /10. The corresponding distri bution of lengths is also shown in the figure. The mean normalized length can be varied by rotating the composite crystal,about an axis perpendicular to the direction of propagation. 77e find that for a given crystal the harmonic intensity would oscillate as a function of the angle of incidence^1 \ Also,in many twinned crystals it would be possible to obtain fairly high conversion efficiencies. However, because of the intensity variation across the cross section of the beam, the highest conversion efficiencies obtainable from a crystal would generally be lower than that calculated for plane waves. Beferences: 1. C.F. Dewey, Bev. Phys. Appl. _12, 405 (1977) 2. J .A.Armstrong e t a l, Phys. Bev. 127. 1916 (1962) 3. Handbook of Mathematical Functions (eds M.Abramowitz and I.A.Stegun) Dover (1965) pp.571. - 142 => Harmonic Degradation of Several Optically B irefrigent Crystals In The Near And Far'Fields K.K. Sharma, M.L. Goyal and G.P. Srivastava Department of-Physics, University of Delhi-110007. In this communication,harmonic(as well as parametric) degradation due to the index inhomogerieities of several optically birefringent(uniaxial and biaxial) crystals have been reported by the authors in the near and far fields of the beam. The effect becomes appreciably important when high pover(short pulsed) repetetively Q-switched lasers are used and the region of study lies such that R^ > 1 and g ' ■ where Ry and ^ are collimation and field parameters of the beam,respectively. This effect is direc tly related to the power absorbed by the crystal per unit volume(provided the absorption is not too large to induce multiphoton breakdown or las#er damage etc) towards the exit face which in terms of field parameters Ry and ^ may. be written by zt relation as: Wav I-exptrjl) j$n l J \ d r r e x p (- T //% Q+i f ] ) where the different terms and parameters have their usual significance(1,2,3). Figure(1) shows quantitatively the variations in w ^ with field parameter ^ at fixed confocal le n g th s b c f within the crystals The spatial index inhomo geneity effect due to-the laser beam within the medium have been brought out by solving' the previous heat equa tion (4) for a semi infinite medium-of length %, absorption coeffecier.t °? and thermal conductivity K a s : z d r 1 + d r , + " _ ( 2 , and by solving it get : dZ _ - M as ■c40+i¥)fl-W(-rfa*G+W%,2) dr 4k L The effective change in phase sensitive harmonic genera ting property of the crystal or 1inhomogeneous phase mismatch1 is then obtained by temperature expansion of the birefrengence term (as in S1IG) and using two - - 143 =■ denensionless parameters a» = W sLOi.fc r t r u e - ( M . ■$"< Pt [T„(<1*)-7,] such that fljgoverns precisely the qualitative absorption within the medium and^ gives a quantitative measure of the changes in the birefringence induced within the r.ediurv The additional 1phas4 mismatch term' for"an"original!y phase matched medium is then obtained directly in terns of these quantities as follows Ak(Ru,,?J -- The increase in the values of and '£/? with ?v have been obtained by the authors for various values of of the" birefrengent crystals and plotted in figures 2 and 3 simultaneously for these crystals. The results show that phase matching properties for these crystals to be essen tially and I® dependent and under high power propaga tion condition shift towards higher temperature conditions' of the crystal. The results as mentioned above are obtained in the near field case i.e-. ?1- and and can be easily extended to the far field lim it. J (Ze-fJ -(6) D In the presence of double refraction the overall deviation in the energy propagation direction in compa rison to the phase propagation direction for an e-ray is readily obtainable by temperature differentiation of the refractive indicies for the o and e-ra.ys simultaneously as indicated above. The harmonic walk off or the aperture length is then obtained directly in .terms of these inho-iogeneous parameters Qj?using (5) ■ The details of these calculations will not be discussed here in this communication,the render may refer(5). These results have now been applied to obtain the effect in UP(harmonic power) conversion effeciency and to study the effects on .'II (harmonic intensity) distribution or fine structure - 144 - variations at the exit face of the crystals. Figure 4 and, 5 describe the drop in the HP conversion efficiency (with a-temperature retuning quantity) per unit increase in the' inhomogeneity parameter within the crystal. The decrease in coherence length or volume can now be obtained from.th* same set of relations (4) and (5) as noted above 2C% ^7 Rujsl-tf a/txi aboui S2°/o- I _ 2 n 6 _ _ _ — — ------— 5 ay e*p (7 ) for Rw = 2.0 and 42% for = 3.4 units, for an ADP crystal using suitable values of the parameters). Die effect-is seen to be more pronounced when birefringence variations duetto temperature for the different crystal classed is also high enough. Cprresponding thermal mismatch may now be obtained under sim ilar condition as stated above and plotted in terms of Rw parameter of the beam. This is shown in figure 3 for the case of near phasq matching at the centre of the bean; sim ilar curves may ndw be obtained for other harmonic generating crystals, depending prim arily on absorption,birefringence effects of temperature and the thermal conductivity of the mate rial. The discripencies between theoretical and experiment tal values of thermal mismatch have already been reported by several authors using low power He-Ne and high power repetitively Q-switched solid state lasers. These results are now compiled by the authors from the present theore tical point of view and presented in table I for a few actively used harmonic generators. These discussions may well be extended to include thermal mismatch in para metric mixing and other higher order processes using focused and unfocused beam where a slight mismatch of phase velocities may detune the ascellator to entirely mow wavelengths. - 145 - table i Crystals AfcH’ . AkT . Afc j 0 t t r s %v Tm KDP C°c) 50.4 ° 4 9 .2 ° (50.4-1.4)° RDP 6?. 0° 6 2 .0 ° (6 7 . 0- 1 . 2 )° Li'NbO^ 4 6 .0 ° 4 0 .0 ° (46.0-2.3)° p a r a BagNal'IbgO^ ^ 8 9 .O0 7 2 .0 ° (89.0-4.1)° REFERENCES: 1. M.L. Goyal, K.K.St^rrna and G.P.3rivastava,Acta Polonica 1980 (accepted for publication). 2. M.L. Goyal, K. K. Sharraa and G.P. S rivas tava Canadian Journal of Physics 198o(accepted for publication). 3 . M. Okada and S. leiri, IEEE Jr. Quant. Electronics, 9 , 1971. 4. U.S. Carslaw, Mathematical theory of the conduction of heat in solids, London : Macmillan 192t,p.12. 5. M.L. Goyal, K. K. Sharrna and G.P. 5 rivas tava, Acta P o lo n ie s 1980 (accepted for publication). - 146 ■» Temperature Tunable Coherent Infrared Generation in Nonlinear Crystals, G.C.Ghosh, P.S.Ghosh and G.C.Bhar Burdwan U niversity,Physics Department Burdwan, 713104, Of the various ways of generating tunable coherent infrared radiation in nonlinear crystals the ones utilising difference frequency mixing and optical parametric oscillation have been of much interest,W hile efficient generation requires phase matching through utilisation of crystal birefringence,the extension of operation of the device to other wavelengths is often realised by appropriately changing the phase matching angle,Since the refracture index is temperature depen dent, the tunabllity of the device can equally well be done through a variation^ of temperature of the nonli near crystal. Such a temperature variation to tune the nonlinear device is convenient without^requiring changes in crystal orientation^ thereby preventing from system )|e S { misalignment,While tbis^been widely used in nonlinear crystals,like ADP,BSN, LiNb©^ in the visible range,such a study is rare in infrared crystils.Bven in the case of an angle-tuned system,a designer for a high power nonlinear optical device should know how much tempera ture stability*of the system is under undesirable rise in temperature of the crystal resulting from absorption of imeenverted input laser radiations through crystal - 147 residual absorption or ambient temperature fluctuations. We have explored the temperature tuning posibility for the generation of tunable infrared radiation with the help of available refracture index from various sources and find that temperature tunability can profitably be t)Sed in some nonlinear crystals for certain wavelength ra n g e . The primary concern in such study ip first to find the temperature coefficient in refractive indices for the nenlinear crystal under consideration.lt is also important to smooth the data taking appropriately into account of the dispersion centres.We have found out a method of analysing such data as obtained from various sources and the analysed data have been presented in a readily usable fornulThis has been verified in predicting tuning behaviour of KDP type of crystals.Since the polarisation amongst the interaction beams is used in phase matching of the nonli near device the more the difference in the temperature coefficient of refractive indices between ordinary and extraordinary polarisations,the larger w ill be the tempe rature sensitivity of phase matching; or in other words, to keep the crystal orientation unaltered under tempera ture change a wider change in the generated wavelength would be produced.A crystal with a low value for tempera ture coefficient for birefringence w ill show poor tempe rature tuning.Pbase matching characteristics are analysed for the following infrared transm itting crystals $ HgS, GaSe^6dSe^and 2iGePg. The latter two crystals possess i father low value for dB/dT as compared to IANbOg and ADP. But even Isi such crystals judicious choice of appropriate pump laser cm yield reasonably wide infrared tuning as shown in the table. Lack ef detailed temperature dispersion data in AgGaSgf AgGaSe2 and CdGeASg prevents us from such characterisation while tunable device based on proustite seems to be fairly insensitive to temperature variations. Within the studied temperature range the crystal is belie ved not to exhibit any structural phase transition.lt is t h e r e f o r e seen from the table that almost the entire median infrared tuning range can be covered by temperature tuning -of appropriate nonlinear crystal. REFERENCES » 1. D.T.Hon t Laser Hand book,Vol.3,Nortb-Holland(1979) 2. G.C.Bbar and G.C.Ghosh s J.Opt.Soc.Am. 69 .730 (197 3. G.C.Bbar and G.C.Ghosh t Appl.Opts. 19 10 29(1980) 4. G.C.Bbar and G.C.Ghosh * IEEE J.Quantum Election IB 8 3 8 (1980) T a b le Temperature - Tuned IR Generation Crystal dB/dT Device Pump/s Tuning range Temperature ( x l o V 0C) (p> ) (ftm) ra n g e (° c ) LilfbO-5 » u npn ru Nd SHG Qe6 _ 5 ig 200 tQ 450 HgS 5 .4 0P0 0-6945 12.0 - I?. 0 -170 to 250 ZnGeP2 1.4 0P0 2*56 3-0 - 11.0 -200 to 200 149 GaSe 15*2 0P0 - DC 1.06 8.0 - 18*0 -15 0 to 100 CdSe 0.4 0P0 2.87 10»5 - 16*3 -200 to 5OO Tl5AsSe5 8>2 OPO DC 2*87 9-0 - 16.0 -150 to 100 LASER PHOTO-CHEMISTRY Isotope Selective Laser photolysis of S-Tetrazine - K.B.Thakur and V.A.Job. Carbon-13 enrichment by IE multiphoton chemistry of CP-Cl-effect of laser frequency, fluence and subetrate pressure on selectivity - Y.Parthasarathy, S.K.Sarkar, A.Pandey, K.V.S.Rama Eao and J.P.Mittal. T3A CO^ laser Induced photodeccoposition of UP, via interspecies V-V energy transfer from multiphoton excited SF, R.S.Karve, S.K.Sarkar, K.f.S.Hama Bao and J .? .Mittal. Eole of Symmetry of stark components in non-radiative relaxation in Eu+^ in Eu-Diglycelate - D.S.Roy, K.Bhattacharyya, A.K.Cupta and M.Chowdhury. High power crossed laser and fast beams set up for the study of photo-diseociation of atomic and molecular species. S.D.Sharma. - 151 - ISOTOPE SELECTIVE LASER PHOTOLYSIS OF 5-TETRAZIKE K.B. Thakur and tf.A. 3ob Bhabha Atomic Reaeareh Centre Bombay-400 085 ABSTRACT During the past feu years separation of isotopes of several elements by selective excitation uith lasers has been demonstrated on a laboratory scale, and considerable effort is being put into adapting this technique for large scale separation of isotopes. The separation of the isotopes, of Carbon and Nitrogen by . the single step photodissociation of s-tetrazine, haS =-rea^Y been achieved 1 -5 both in the vapour phase and in the condensed phase . We have undertaken a detailed investigation of the tetrazine system to assess the feasibility of large scale laser isotope separation as moat of the requirements for an ideal system are met in the case of this ■ m o le c u le . 13 12 14 Natural s-tetrazine molecules contain 2.1% C C N H2« 12 15 14 1 ,4%C2 N H2 and much smaller amounts of deuterated species and species uith more than one heavy isotope. The Q-branch uith a sharp edge in the visible absorption spectrum and the isotope shift of 3 cm- 1 for the 13C and 13N speciee enable us to excite the abundant species uith a very high degree of selectivity even uith a coarsely tuned laser and operating in a -multimode configuration. Because of the short dissociative life time of 0.5 ns isotope - 152 - scrambling processes are entirely eliminated between selective excitation and dissociation. A vapour,pressure of 1 torr at room temperature, high absorption coefficient and large quantum yield (0.999) for dissociation ensure efficient utilization of the laser power, and a. large throughput. The s-tetrazine molecule was synthesized from glycine by the 6 method described by Spencer et al . The mass spectrum of natural tetrazine as well as laser irradiated tetrazine samples were recorded on Extra Nuclear Labs. Inc's EPIBA II quadrupole mass spectrometer. The visible absorption spectrum of tetrazine was recorded on a 3obin-Yvon THR-1500 monochromator. Selective dissociation of the light tetrazine (mass 82) was carried out by irradiation with a dye laser (sodium flourescein dye) pumped by a Solectron 1 Mii pulsed nitrogen laser. The laser line width in the in itial experiments was - 1 0 . 6 cm and the laser was tuned to the absorption maximum of the abundant species by visual monitoring of the flourescence. Further experiments were carried out with a fVe. Coherent Radiation's CR-590 CU dye laser pumped by CR-18 Argon ion laser. The irradiation wave length was fixed at 5513.9 A and a narrower laser line width of 0.3 cm ^ could be used in this case. The experimentally determined ratio of the light (mass 82) to heavy (mass 83) tetrazine in natural sample was 26:1 (expected value 26.6:1). After irradiation with the pulsed laser this ratio changed to 5.5:1 in the undissociated tetrazine. A much higher enrichment was obtained by Cltl irradiation because of the narrower laser line ~ 153 - width and store accurate tuning. The ease 82 paak could not be detected above the back ground in the aaea spectrue, and the enrich ment factor was well over 1000. Even though a high degreee of selectivity can be achieved with narrow laser line width, this is at the expense of laser power. For a large scale separation scheme it ia necessary to determine the optimum irradiation conditions, With this in view we have estimated from simulated band contours the relative absorption coefficients of the light and heavy tetrazine species for. various irradiation wavelengths and laser line widths. The throughput, power require ments, yield and selectivity that can be achieved under various irradiation conditions will be discussed. We are indebted to P.L. Pandey, A. 3oshi and A.K. Talukdar for assistance with the operation of the lasers and mesa spectro meter. We gratefully acknowledge many fruitful discussions with Dr. P.R.K. Rao and Or. V.8. Kartha. 1) A.K. Karl, K.K. Innes Chem Phys lett 36, 275, (1975) 2) A.R. Hochstrasser 6 D.S. King. 3. Am. Chem, Soc. 98, 5443 (1976) 3) Davids, King et al 3. Am. Chem. Soc. 99, 271 (1977) 4) V. Bosel et al Chem. Phy. letter 61, 57, (1979) 5) " " " " n 61, 62, (1979) 6) G.H. Spencer et al 3. Chem. Phys 35, 1939 (1961) - 154 - CARBON-13 ENRICHMENT BY IE HOLTIPHOTON CHEMISTRY OP CPa Cl - EFFECT OF LASER FREQUENCY, FLUENCB AND SUBSTRATE PRESSURE ON SELECTIVITY V. Parthasarathy, 3 . K . Sark&r M ultidisciplinary Research Scheme, A. Pandey Laser Section, K .v .S . Rama Rao and J a i P . M itta l Chemistry Divi 3 ion Bhabha Atomic Research Centre, Trombay, Bombay 400085 1. Introduction lliere is much current interest in the IR multiphoton disso ciation of CF3X (X = C l , Br or I) system for carbon-13 enrichment. Among these, CFgCl and CF3Br offer better isotopic select!vities at room temperature. A dramatic increase in the selectivity factor *oC • with increase of substrate pressure has been reported [1] for CFgBr under low fluence, "off-resonant" irradiation. We report findings on carbon-13 enrichment in CFgCl photolysis as a function of laser frequency, fluence and CFgCl pressure to verify the generality of the above unusual effect of pressure. 2. Experimental A line tunable, helical TBA-COg laser -built in Laser Section was used. The laser beam (pulse ener@r ~750 a j , diameter 1.4 cm) was fo cu sse d w ith a BaF3 lens. = 15 cm) and depending on the fluence requirement of the experiment, the pyrex reaction cell (9 cm long, 5 cm dia.) was positioned with the focal spot either at the centre of the cell (strong focussing, focal fluence ''■’500 J/cm3) or 3 cm away from the exit KBr window (weak focussing, average fluence ~3 J/cm3) . Yield of reaction product, C2F6, was monitored by IR spectrometry. C-13 enrichment factor in CaPa was determined by mass spectrometry. In order to choose the laser line for "off-resonant" irradiation, absorption measurements for CFgCl in 9.6 Pm region were carried out with a low-power ( '-'I W), line tunable CV-COg laser. 3. Results and Discussion 3.1 Near Resonant and Strong Focussing Excitation Fig.l shows the selectivity factor (<) and CaF6 yield as a function of laser frequency for 1 torr CFgCl under strong focussing condition. When the laser line is progressively red-shifted from !3CFgCl band c e n tre (1077.4 cm"1) , s e l e c t i v i t y i s found to increase continuously but with decrease of CgFg y ie l d . As ex p e cted , < was reduced at higher CFgCl pressures in strong focussing condition. 3 .2 Off-resonant and Weak Focussing Excitation For these studies, P(l8) line of 9.6 Pm band at 1648.66 cm"1 - 155 - (off-resonant by 29 cm-1 for tiCFgCl and 58 cm-} for CFgCl band centres) was chosen because of favourable absorption cross section. The average fluence employed was 3 J/cm2 . Typical results are given in Table I. These data clearly indicate that oc increases with pressure, reaches a maximum and then drops down to 4.29 at 20 torr. We have observed this effect in a somewhat smaller pressure, range compared to an extended range reported by Gauthier's group. This is understandable in,terms of differences in beam optics. Gauthier et al have used an almost parallel beam of average fluence of J/cm2 whereas our beam includes regions of higher fluence because of converging geometry. There is a clear indication that selectivity vs pressure is a strong function of excitation fluence. This is demonstrated by,, the drastic fall in isotopic selectivity from 45 to 4 at 4.5 torr pressure, when weak focussing was changed to strong one for the same off-resonant excitation. 4. Conclusion In summary, we have carried out 0- 13 enrichment by IR MPD of CFgCl for various laser frequencies. We have shown that isotopic selectivity continues to increase with CFgCl pressure upto 10 torr in off-resonant, weak focussing excitation. Reference (.1] M. Gautheir, W.g. Nip, P.A. Rackett and C. Willis, Chem.Phys. Lett. 69 (l98"0) 372 0-20 b 8 > u 7 ui ' -J UJ in R-BRANCH. CO; LASER LNE (9-6 t^n bond) „Pig.l. Frequency dependence of < and C3P6 yield. 31b at rate pressure! 1 torr; incident fluencei 0.5 J/cm?. — 156 T able I Pressure dependence of «C* values under off-resonant, weak focussing excitation P re ssu re No. of pulses «C CgPg y ie ld (b) ( to r r ) ( a to r r ) 4.5 2000 .45 ~ 1 10 2000 65 ~ 1 - 2 20 2000 4.29 's/3 4.7 2000 4.03 15 (a) In this experiment off-resonant, strong focussing irradiation was done. (b) Yield of CgFg was based on mass spectrometric signal though Cq Fq pressures were drastically modified by a large excess of CFgCl, ISA C02 LASSE n n r a c s s FHOTOEBCOMFOSITION OF TJFg.v i a in t e r - SPECISS V-V ENERGY TRANSFER FROM MUITIPHOTON BXCITBD SFg R.S. Karve, S.K. Sarkar M ultidisciplinary Research Scheme and K.V.S. Rama Rao, Jai P. M ittal Chemistry Division Bhabha Atomic Research Centre Bombay 400085 Introduction? Infrared active vibrational modes of TJFg lie in a spectral region which is not accessible by 10*6/-CO. laser. However, it occurred to us that C02 laser multiphoton absorption may be sensitized in TJFg by in itially exciting TJFg into its vibrational quasicontinuum by vibrational energy transfer from SFg which is a good absorber of C02 laser radiation. We may make use of V j mode of SFg (947 cm"* ) to absorb the resonant CO- laser photons and take advantage of inter mode relaxation to populate its V . mode. The "V^ mode of SFg (617 cm” ') lies very close to the V , mode of TJFg (625 cm”^)« We may therefore expect a rapid vibrational energy exchange between SFg and TJFg via and modes respectively. Once the TJFg molecules are excited to a few quanta in this manner, a quasi conti nuum state of TJFg is evolved and direct absorption of 00^ laser photons and photodissociation of TJFg becomes p o s s i b le . Experiments and Results The laser used in our experiments was a TEA C0„ laser which was built by laser Section, BARC. It has a typical pulse width of 500 ns with a pulse energy of 250 mJ. A 10 om long nickel cell provided with end KOI windows was passivated and filled with a mixture of SFg and TJFg at a partial pressure of about 20 torr in each component. The mixture was kept in the cell for three days to check for any dark reaction. No depletion in either constituent occurred during this period. This fact was established by monitoring the IR spectra of the mixture. The sample was then exposed to COg laser radiation. The laser beam was focussed off-axis with a 30 om focal length concave mirror. The cell was posi tioned such that the laser beam converges through the cell but gets focussed only after the beam emerges out of the cell. This optical arrangement was employed to avoid any - 158 - dielectric breakdown of the sample which would occur if the focal spot lies within the cell. Typically 500 ,,ulses were given to the sample. IR spectral analysis of the sample after irradiation showed a considerable deple tion of UFg. There was no change in SFg concentration. Laser irradiation experiments with single component systems, i.e . SFg or UFg alone at 40 to 80 torr, established that no decomposition occurred under the fluence conditions employed in this work. In the next series of experiment various compositions of SFg and UFg mixtures, in the range of 1 to 20 torr, were irradiated. The results clearly demonstrated that UFg gets decomposed only when sensitized via SFg. Discussion; The SFg pressure range chosen in this work is such, that multiphoton absorption of COg laser photons is lim it ed to well below SFg decomposition levels due to rapid intra and inter molecular V-V relaxation processes. The relaxation time for V ^ intezmode relaxation in SFg is reported to be 1.1 /*s-torr. The relaxation is even faster for higher levels of excitation, e.g., the relaxa tion time for 3^^ level is only 150 ns-torr. The V-T relaxation process is much slower, T a 150 p s - t o r r . At 20 torr SFg pressure, we can therefore expect excited SFg to have reached a vibrational equilibrium within about 10 ns, the V-T relaxation requiring about 7500 ns. In other words V-V relaxation of SFg would be complete very much within the 500 ns laser pulse width which would be too short for V-T relaxation. This would have two immediate consequences, firstly , to keep the multiphoton build up in SFg under check and secondly to preserve the excitation energy within the vibrational modes of SFg for possible exchange with UFg. Intermolecular V-V energy transfers between SFg and other polyatomic gases such as CHOlFg, and CgFg a r e r e p o r te d to r e q u ir e o n ly a few c o l l i sions in contrast to thousands of collisions required for V-T relaxation. Although no such data is specifically available for SFg-UFg mixtures, in view of the closeness of (of SFg) and V^(of UFg) levels we can expect efficient V-V exchange between SFg rod UFg within a few tens of nanoseconds after the absorption of first laser photons by SFg. In other words, the time scale for excit ing UFg molecules into quasi continuum would be about the same as intermolecular V-V relaxation time which is expected to be 30 to 40 ns at 20 torr SFg tod 20 torr UFg. The possible physicochemical processes can be summarised as - 159 - Primary excitation Z'Yl'^ K/ * . 3,6 tiTS-TT -d 5 7 m # S ,6 Intermode relaxation s f 6*(V 3 ) > s f 6* (V 4) Energy transfer SFg (^4) + ^6 ^6 (^3) + SFg The actual mechanism for decomposition of TJFg may involve two alternative or concurrent processes. The first possibility is that the above three steps would eventually build up enough vibrational energy in UFg for its decomposition via V-V energy transfer from SFg. 2 However, it appears that at a fluence of about 1 J/cm , a s in our experiments, the multiphoton excitation in SFg is limited to about 10 V- level i.e. -^1 eV. On the other hand, one is tempted to suggest that the above V-V transfer would be able to populate enough UFg molecules into vibrational quasi continuum. These excited UFg molecules w ill now be able to directly absorb COg laser photons leading to their dissociation, e.g., * , . Z->\> IxV ** UP,- ( "V•*) » UFz- — UF, + F. 6 3 C02 l a s e r 5 The only evidence we have at present for such multiphoton absorption is the requirement of focussing the laser beam to initiate UF, decomposition. No decomposition was observed without focussing. Nevertheless, more detailed evidence such as real time measurements is required to iclearly distinguish between the two possibilities. In summaryj it is shown that UFg can be photo- dissociated with C02 laser (10.6/-m) radiation, if sensi tized in the presence of SFg. We believe this could be ,due to a two step process. In the first step UFg mole cules are populated to the quasi continuum state via intermolecular energy transfer from excited SFg. The second step involves-the absorption of OOg laser photons by such excited UFg. - 160 - Reis sf Symmetry ef Stsrk Components In He'n-radlatlve Relaxation In Bu*^ In Eu-Diglyeelate Deb Shankar Roy, Kankan Bhattacharyya, Anlndya Kunar Gupta and Mihlr Chowdnury, Departmant ef Physical Chemlatry, Indian Aeeoclatlen fer the Cultivation ef Science, Jadavpur, Calcutta-700 032, INDIA In spite of great dea). ef efforts Hade In understanding the nen-radlatlve relaxation, our knowledge e f the re le ef electronic symmetry In the Internal conversion process le rather peer. The overall symoetry le expected to be conserved In a radlatlenleee transition) however, this concept can hardly be u tilise d for making any jfredlctlen In the case e f large organ!c molecules, fe r one can form numerous near degenerate re-vlhrenlc states of different eymnetrlee from one electronic state ef a particular symmetry fer any moderately-siled molecule. With the hepe that some worthwhile predictions can be made from electronic symmetry ef etateo In the ease ef simpleet rare earth crystals, we have carried eut a time resolved study ef emleelene from "*D^ levels ef Eu*^ In EuvJlglycelate after selective excitation ef "*D^, ^ and ®D^ states with a pulsed dye laser. The study has revealed the d iffe re n tia l behaviour e f CF components leading te by-passing ef levele In the nen-radlatlve relaxation process. When any one ef CF components of (which In D^ CF s p lits Into Ag t£$8) Is excited, the endsslon occurs both from ^D# end ^D^. The decay (tc ~ 1.2 mse exhlhlts a fast growth having a rise tine .6/^eec, fallowed fay a decay - with time constant 1.45 vaee. The Initial fast rise ef % can be 5 explained aa a part ef the decay ef D, but net the slower one) which 5 5 must be due te a d ire c t tra n s itio n firem to D^. However, a simpleton twe channel mechanism (a) tflaet) and ^ b (el**) w ill essentially lead te one exponential grewth; The m ph channel would be the faster ef. the twe rate processes. This leads us te the conclusion that twe decay nodes ef must be Independent, arising firem twe dlffrent levels * 5 The twe widely different rates of grewth ef can be explained by the following mechanism. Following excitation ef E(^D-), the energy degrades - by twe fast competitive channels - one te and the ether te a trap level; the trap level then preferentially transfers the energy te • Energetically It Is Impossible for the triple state of the organic ligand te act as a trap level. Nor could we find any evidence fer an Impurity BE lea level acting as a trap. Zt is hard to think ef crystal imperfect!one acting as a trap fer a narrow specified energy range. We, tharefere, believe that ef CF components ef ^D,, la acting as the intermediate trap level. In ether werds, following an excltaflien at E(^Bg) a part ef the excited species is non-radlatively deexcited te at a rapid rate via while a significant 5 ' amount, through a very fast process, populates Aj *f which acts aa a trap preventing deexcitatlen ef via and th6e inducing S slew direct level by-passing transition to"^ (A^). On excitation te CF components of , the ealssien could be 5 * 5 observed from and 0#. The bex>car averaged emlsslen intensities ratio ef ^D#/ was mere than obtained from excitation and less than that from ^0^ excitation while the grewth ef has been found te be tee fast to be detected by experimental means, the gnjwth ef has a rise 5 time ef 2 /“-see. These facts mean that Dg is effectively by-passed. The differential behaviour ef the twe Cl components can be—- treated considering the model ef Orbach, Scot and Jeffries, where the HE ieo Is considered te be embedded in a t lattice whese vUssatlens are represented by phenen system. The tra n s itio n p robab ility fe r a tra n sitio n e f combined len-phenen system te 1 . S-Hv is expressible In terms ef the square ef perturbation matrix element. _ - 162 = I v ^ i r i ,«*-#>> V" te the p-th order term tn the CF expan el en, * , n being the average. P phenon ne# Pellewing Or bach medel i t te peaelble te shew th at the abeve matrix element le the preduct ef an electrenlc part and lattice etrain parte i | r i m < • i <* I »»►> Since eheuld beleng to A( eynanetry we are led te a 1 seft selection rule* that the treneitiene between etark cenpenents ef identical eynmetry are aliened.' Thle explain the nan-radiative ferblddenneee ef -—? ^>2. ar*^ allewedneea ef ——> ^D#(A£) transition# - 163 - HIGH POWER CROSSED LASER AND FAST BEAMS SET DP FOR THE STUDY OF PKOTODISSOCIATION OF ATOMIC. AND MOLECULAR SPECIES S.D.SHARMA Tata Institute of Fundamental Research Colaba Bombay-400 005 Photodissociation represents the primary initiating• step in a variety of photochemical reaction systems which are important in laser isotope separation,chemical synthesis,photochemical lasers,chemistry of upper atmosphere ana planets and formulation of theories of reaction dynamics. The studies of photodissociation of positive ions using fast beams and high power lasers are relatively very new and in their infancy. The experimental arrangement to be described here is with reference to the one at the Physics Department of the University of # Pittsburgh at Pittsburgh U.S.A. Figure 1 is a schematic of the apparatus. The rf discharge ion source with a beam energy spread of 20 eV fwhm can produce a wide variety of excited atoms, ions and molecules in varying amounts. The atomic or molecular ions can be accelerated upto 30 keV energy with a linear accelerator. After proper collimation, the ion bean of interest of particular mass and energy can be selected by the analyzing magnet and. passed through a charge exchange; rp.s target (usually Argon) fo r collision- production of CAMfiC /V7 F/L7E4 Sys-rsM FAST ATOMIC BEAMS APPARATUS A 'd r l e h s > A ,6 " » .P . ATOM DETECTOR MICROWAVES [ I METEg ^ & electrostatic ION ION ANALYZER Z y^Sc PUMP Qf>lWST£K I » eiECTSlCAU ISOLATION SECT 10// n i n Doh I CRYOPUMP r~i m i—i ^ CD cffiT/C E>-EC. fielzs — As GAS 4 00 keV TARGET I jMf«r. van DE GRAFF 6 D.P. ttpn ' TPM5P/C/’i leem v - >' i o " d . P mlRKjP "ANALYZING MAGNET 6 " D.P. JELATfi PUIS E T sokev ion accelerator E*WKfE£ • -EEZ>C3--£ PlfJHOLE PA cw CO, Laser FILTER LUMOP//CS if?/pulse F/&. i C03 TEA LA SEE - 165 - various long lived excited states of neutral species. Even negative ions can be produced through a double charge exchange. Next, the beam passes through the •interaction region v/here the laser pulse crosses it at right angles. The charged photodissociation products can be electrostatically analyzed and counted. The high notier laser pulses (>200 MW/pulse ) are obtained from the Limonics 151, 2 TEA. system, acting as an amplifier. Before entering, the amplifier, the outnut from a T ach isto TAG TIG g ra tin g tuned CO 2 laser oscillator is passed -through a pinhole filter and beam expander fo r b e tte r beam q u a lity . A microcomputer-'controlled CAMAC system controls the important variables ana events in each experiment. In this case,both the oscillator and amplifier laser pulse triggering,fast pulse electronics, laser energy meter and data acquisition are under its primary control. The oulsed laser data can be catalogued and analyzed on-lind by the computer tihich has been equipped with an a d d itio n a l memory and understands a pow erful extended X-Y basic language. The above cescrived apparatus has been used to study the nhotodissociation of ions as a funtion of laser power. The choice o f H 2 ion vias m otivated by the siranlicity as well as importance of the system in view of the attempts to,.'achieve thermal fusion of "'ycroren 166 - in the laboratory and the advancement of Astrophysics. It is increasingly being realized now that the molecular, stucture in the intense fields 1 can be quite different from that with no field. The present study was hence undertaken with the view of learning more about the system under non-resonant oscillating electric fields. During this experiment, the proton signal obtained from the product of photodissociation of a 8 keV H2 beam was observed as a function of laser pulse energy. The laser induced process is of the following nature (Hg) V hv -* H+ •+ (H) The preliminary findings suggest' that the variation of dissociation rate is as shown in Fig.2 upto the laser ' energy o f '- 3 Joules per pulse for any fixed frequency. ■p § o o + Lase oulse energy (Joules) F in . 2. Rczeronce: 1. J.^.-ayfield, Physics Reports 51(1?7D)317‘ v The author was with the Physics Department of the university of Pittsburgh for a year when an important portion of the setup was completed. The work renorted v s done with Professor J.E.Tcyfielc in his . • . 1 ' horr.tory. ' LASER SPECTROSCOPY Energy transport studies 'using laser 167 excited spatially resolved luminescence in 3aP:H - S.C.Abbi and C.Hirlimann. Ion-pair relaxations and energy up- 170 conversions in Lai’^iSd^+_ B.H.Heddy and P.Venkateswarlu. Laser excitation and fluorescence 174 spectra of _ D.N.Bao, D.R.Bao and P.Venkateswarlu. Trap parameters in CaS and ZnS doped 176 phosphors using H„-pulsed laser - H.S.Bbatti, H.V.tJ.Nair and H.D.Singh. Diode laser spectroscopy of isotopic 180 EH, - H.D.Patel, B.D'cunha, A.Joshi, V.B.Kartha and V.A.Job. Two step excitation technique for 183 dissociative spectroscopy of Nau- moleeule - H.P.Borgaonkar, A.S.inamdar and H.Mehendale. Two photon absorption technique for the 186 study of de-excitation mechanism in sodium atom - A.S.inamdar, H.P.Borgaonkar and H.Mehendale. Parametric conversion in resonantly 189 excited potassium and sodium vapours - R.Bhatnagar. Laser Induced intensity changes in 193 atomic lines of uranium excited in a hollow cathode - S.D.Saksena and A.K.Trlpathy. = 16? - ENERGY TRAILS PORT STUDIES USING LASER EXCITED SPATIALLY RESOLVED LUMINESCENCE IN GaP:N S a tis h C. Abbi Physics Department Indian Institute of Technology Hauz Khas, New D elhi - 110 016 and C. EH RLI MANN Laboratoire de Physique des Solids UnLversite Pierre et Marie Curie L Place Jussuien, Paris Cedex 05 FRANCE Laser beams are often focused on small areas of d iam eters 1 5 - 5 0 j u n t o obtain intense illumination and temporally analysed. In many luminescence studies, especially at low temperatures, the crystals are seen to luminesce from areas adjoining the laser illuminated area but this luminescence is often not studied carefully. We give some experimental results concerning this luminescence in GaP crystals with low concentration of diffused Nitrogen ,in it, to show that spatialy resolved luminescence studies can give a variety of information concerning the role of excitons, optical phonons and acoustic phonons in the energy transport processes in this crystal., The method of spatially resolved luminescence described here can be applied to study energy transport processes in a variety of crystals. A variety of experimental results concerning spatially resolved luminescence from GaP samples doped with Nitrogen have been obtained and some representative results are shown in Elg.1. The inset of Eig.1 shows the lumine 168 - scence s p e c tr a o f th e (100) face o f GaPrN c r y s ta l w ith -n'^ = 5 x 101? cm ~ ^ i The spectra was recorded at 95°K and at this temperature shows various lines which can he 1 -? identified using known studies on the luminescence ppectra of such crystals. Line A is related to the recombination exdtons bound to single Nitrogen sites. Line C is the LO phon ( A^ = 402 cm"1 ) repelea of the line A. Line B is the TO acoustic phonon repelea of Line A, line D is a similar repelea of line C. A spot of 2 0 diameter was illuminated by laser light and lumine- scence spectra of these lines was spatially resolved and the results are given in Fig.l. The large scale exponential decay of luminescence can be used to define a scaling length parameter through the equation: _ x / a I (*) = I(»> 6 ' The physical meaning of the characteristic diffusion le n g th A » its dependence on temperature and phonon mediation in energy transport processes needs to be further investigated. References: 1. D . g . Thomas, J.J. Hopfield and C.J. Frosch Phys. Rev. Lett 15 (22) 8?7 (1965) .2 . D . g . Thomas, J.J. Hopfield, Physical Review 150 (2) 680 (1966) 0.01 0.1 I(au) e .25 0 SPECTRA LUMINESCENCE mm) m m < x 0.5 6 - 169 - 0.75 18500 1.0 170 - IO ti-P A IB BRT.AX a T IU M S Ai'.B M B d u T U P-G O illfiiB SlutiS IB LaF, t Bd3+ 3 B.E. Beddy and P. Venkateswarlu Lasers and Spectroscopy Laboratory Department of Physics Indian Institute of Technology Kanpur-208016 The spectra of B.E. ions doped in crystals have been studied extensively.1 An ion in the excited state can come down radiatively or non-radiatively. The non- radiative decay rate of the ion depends on the host in which it is embedded. The-ion may*lose its excitation energy by (l) Phonon/multiphonon emission, whose effici- eney depends on the energy gap to the next lower level and on the frequency of the cut-off phonon of the host lattice or (2) by energy transfer to the neighbouring ion(s) of sim ilar/dissim ilar kind, and the efficiency - o f _ the transfer rate3 depends on the overlap and nature of the Ion-pair transitions and on inter-ionic separation. The present paper briefly reports some of the energy transfer results we have observed in LaF^tNd3* fluores cence spectrum. A series of single crystals with Bd3+(G.l,. 0.5, 2.0 and 3.0 percent by wt .) doped in LaF^ have been procured from Optovac Inc. The crystals are irradiated with Molectron #2 laser pumped dye laser. Fluorescence is recorded using a 0.75 m Jarrell-A sh spectrograph fitted with an ITT PWL30 PMT and pi coammeter and stripchart recorder combination. Decay times are measured by giving the output of the PMT in sequence to an em itter follower, am plifier and an oscilloscope. The energy le v e l diagram of LaFj*Nd3+ shown in Fig.1. On resonant excitation of the D levels we have observed fluorescence from B, I?, D and -. =■ 171 - | ' M- Flg. i. Partial energy level diagram of laFjiNd showing the observed fluorescence groups al room and liquid nitrogen temperatures when D levels are resonantly excited- (» observed only at RT ) 5335.5 18737 5301.6 18857 5276.5 5 2 59.0 19009 5232.2 ° 5773.4 tz: - 173 - up-converted fluorescence from E, K and L levels and (* observed at room temperature only) are marked in Pig.1. Excitation spectrum has been recorded for at least one fluorescence line of each group. A typical fluorescence (B-Z group) and excitation (of 5232. 2 50 spectra are 1 shown in Pigs. 2 and 3- The excitation spectrum contains transitions from Z^ and Zg to and ^ only. It implies that only D^ and levels are effective in populating the E levels. This is, because, the higher levels relax very fast to the lower, ar levels. Prom the fluorescence intensity and decay time measurements we have found the methods of population^ of R, S, E and L levels. The R levels are populated significantly by ion-pair relaxation ( D+Z = R+W). The measured decay times indicate that the population "of S levels is significantly due to thermal contribution from R levels. Two processes, a sequential two photon excitation (STEP) and an energy transfer up- conversion (ETU) have been suggested for the population of B levels. The measured decay times indicate that the energy transfer up-conversion (ETU) is effective in populating the 1 levels (DfR = L+Z ). R eferences 1. G.H. Hieke : Spectra and Energy levels of Bare Earth ions in Crystals (Wiley-Interscience, E.Y. 1968). 2. M.J. Weber, Phys. Rev. 152, 271 (1967). 3. J.C. Wright,1Up-conversion and excited state energy transfer in Bare-earth doped materials' in Topics in Applied Physics Vol. 15, Ed. F.K. Pong .(Springer-Verlag Berlin, 1976). 4. B.R. Reddy, Ph.D. Thesis, IIT Kanpur (Submitted,1980). - 174 - LASER EXCITATION AND FLUORESCENCE SPECTRA OP CaF2 :Sm5+ IX Narayana Rao, IX Ramachandra Bao and P. Venkateswarlu Lasers and Spectroscopy Laboratory Department of Physics Indian Institute of Technology Kanpur 208016 The spectra of trivalent rare-earth ions in CaFg matrix are complicated due to the presence of a number of non-equivalent impurity ion sites which are a result of various charge compensating configurations that are possible. The development of the selective laser excitation methods in recent years has simplified 1 2 the problems to a considerable extent. ’ The method allows the separation of the luminescence spectra due to different centres by selectively exciting the absorption levels of each centre separately. The energy transfer between the levels of the different centres has been observed to be negligible. This method has been success fully used to interpret the spectra of single ion and cluster sites, as well as energy transfer processes in various rare-earth doped svsterns of the type CaF2« We report in the present paper the single ion site spectra of CaF,,:Sm^+ at 77°K obtained by the method of selective laser excitation. A tunable pulsed dye laser pumped by 1 MW pulsed nitrogen laser has been used as the excitation source. The fluorescence and excitation spectra are recorded using a spectrophotometer assembly consisting of 0.75 m monochromator and a photomultiplier tube. The excitation wavelengths were calibrated against tne standard spectral lines of he-he, Ar and Xe low pressure discharge tubes. The fluorescence decay times which were used in the classification of different sites were measured wita a signal averager. Single crystal of Oaf da^+ (with concentrations of 0.ul - 5 percent of Sm - 175 in CaPg) were grown in a vacuum furnace using Bridgeman tech n iq u e. The e x c ita tio n and flu o re sc e n c e s p e c tra ,CaJ?„:Sm^+ in v o lv in g th e S tark m anifolds ^Hc 6h 6 0 6tt 4g 4x 4t 5 '4 t ■ ' 9/2t nj./2’ 13/2’ 5/2' 9/2' -4.1/2' 4 l3/2 . and * 5/2 11376 1)6611 obtained by recording the spectra at 77°K laser. The spectra show the presence of two centres A and B. The classification of the observed transitions has been confirmed by the measurements of decay times. The energy level positions of the various Stark manifolds studied have been obtained for centre A The observed number of transitions indicate that the centreA.is probably non cubic. The spectra obtained with other concentrations show that the transitions due to other centres are negligible in the range of concen trations studied. The intensities of the transitions due to centre B have been found to increase with concentration relative to that of centre A. R e ferences 1. D.H. Tallent and J.C. Wright, J. Ghem.Phys. £2, 2074 (1975). 2. M.P. M ille r, D.R. T a lla n t, F .J . G ustafson and J.C . Wright, Annal. Chem. 42, 1479 (1979). - 176 - ; TRAP PARAMETERS IN CaS & 2nS DOPED PHOSPHOBS USING No - PUISED LASER H.S. Bhatti, Unnikrishnan Nair N.V. & R.D. Singh Department of Physics, Maharshl Dayanand University, Rohtak- 12V 001. A system has been developed to study the relaxation■ processes in optical and photoconducting materials using pulsed riitroger: laser. The decay processes in a number o f CaS and znS phosphors doped with d if f e r e n t a c tiv a to r s have been studied. The narrow pulsewidth associated with high power output helps in finding out a number of shallow Put. SCO Nv- L Aseit PM " tu b b Fig. 1 Schematic diagram of experimental arrangement for lifetim e measurements. traps not detected so far. A number of phosphors doped with killer impurities have been ^ound to be having shallow traps. . The, nitrogen laser used for the excitation of different phosphors had the characteristics of 50 pps with the pulsewidth of 10 nsec duration. The peak power .-xu‘ru t. was. 200 KW a t 3371 *A with beam dimension 5x20 mm. n>.: '-tidrt;period phosphorscence of different energies w-v .etccted with a RCA 8053 PM tube through a constant at-, tier, spectrograph as a monochromator. The output of the i"X' vu.de res fed to a storage oscilloscope OS 763 S and the ^cay curves were photographed by a Polariod camera mounted on it. In order to cut off the reflected UV laser light, a thick glass slab was placed in between the sample and the monochromator. The schematic diagram 177 in d ic a tin g the experim ental arrangem ent has been shown in F ig. 1. RESULTS & DISCUSSION: A representative decay curve for CaS:Cu has been shown in F ig. 2. Since our study is main3y confined to the short period phosphorescence, the asympto tic portion of the decay curves has been neglected. Also because of the narrow pulsewidth of the laser source used, the time of excitation has not been taken into account while analysing the decay curves. The probability 'p' of an electron escaping from a trap(1) is given by p = S exp (-E/kT) where the activation energy E is called the trap depth and S is the escape frequency factor (^10° sec )• Table-1 Phosphor Lifetime Values(Msec) Trapdepths(eV) Decay (0.1%)(0. 1%) » ■ » ■=. c consto n stt T T T e 3 ■ 1 T 3 E1 E2 'b ' CaS :Cu 4.07 18.63 90.00 0,.215 0.254 0.295 0.80 CaS " A.ff 3.86 12.11 2 3 .0 5 0,.213 0.243 0.260 0.47 CaS :M*n 2.88 4.95 10.4 5 0,.206 0.220 0.239 -0.62 CaS :Fe 3.46 10.87 2 0 .3 7 0,.211 0.240 0 .2 % 0.76 CaS :Co 2.89 4.33 15.06 0,.206 0 .2 1 6 0.249 0.64 CaS :Ni 3.00 8.91 3 2 .2 3 0,.20 7 0.235 0.268 0.68 CaS :Mg 3.01 10.43 31.14 0,.207 0.239 0.268 0.55 CaS :Sr 3.24 4.62 9.62 0 .209 0.218 0.237 0.61 CaS :Ba -3A3 9.14 31.01 0 .210 0.2 36 0.267 0.80 CaS :A1 1.83 4.87 31.20 0 .194 0.220 0.267 0.70 CaS :Bi 2.89 4.27 17.19 0 .206 0 .2 1 6 0 .2 5 2 0 .64 CaS • Ce 0.80 5.10. . 12.96 0,.173 0.221 0.245 0.83 ZnS :Cu 1.89 4.97 2 1 .40 0,.195 0.220 0.258 0.82 2nS :Ag 1.46 4.77 7.64 0 .188 0.219 0.231 0.93 ZnS :Mn* 0.08 0.20 0.99 0 .292 0.315 0.357 0.95 ZnS :Fe 1.62 3.73 15.87 0 .191 0.213 0.250 0 .9 0 ZnS :Co 1.38 3.01 2 0 .3 2 0 .195 0.207 0.256 0 .9 1 ZnS :Ni 2 .1 8 4.6 4 11.48 0 .199 0.218 0.21*2 0.78 ZnS :Mg 2.49 3.47 8.56 0 .202 0.211 0.234 0.89 ZnS :Sr 2.38 4.27 8.69 0 .201 0 .2 1 6 0.235 0 .7 4 ZnS :Ba 2.07 3.97 11.18 0 .197 0.214 0.241 0.96 ZnS :A1 2.46 4.31 13.10 0 .202 0 .2 1 6 0,245 0.87 ZnS :Bi 1.83 4.96 12.31 0 .194 0.220 0.244 0.97 ♦Values in m illiseconds. Thus, by studying the decay processes, one can find out the depth of various traps and their distribution. - 178 - Fi<).3 . PEELING - OFF THE DECAY CURVE OF ° -i IO 10 3-0 5-0 ■‘i'o? i'ig 2'. Please see plate D at the end of this book. - 179 - During the present course of investigations, hyper bolic decay curves have been observed in all the cases. The exponential components of the hyperbolic decay curves were obtained by the peeling-off method ( 2 ) as shown in Fig. 3* The shallow traps which contribute significantly to the phosphorescence decay have been found in the range 0.173 to 0.295 eV in CaS doped phosphors and 0.138 to 0.357.eV in ZnS doped phosphors. In general, CaS doped phosphors showed better intensity .than .the corresponding ZnS doped phosphors. So, in orceh to record the decay curves, gain of the PM tube was kept higher in the latter case than in the former ones The one to one correspondence of various dopants in the two lattices indicates that the trap depths are, in general, smaller in case of ZnS doped phosphors than CaS phosphors with the exception of zn§:Mn. An interesting feature of the analysis seems to be the fact that all the 'XT values measured are in the microsecond region, with the exception of 2nS:Mn, where the lifetime values of the order of milliseconds were obtained. Another interesting feature of the present investigation is the decay of phosphors doped with killer impurities (Fe, Co, Mi) (3- 6 ). We note that the trap depths contributing to the phosphorescence in these cases, are in general, sma ller than the other cases. The detection of shallow traps during the present study, is attributed mainly to the use of high‘power short pulsed Kg-laser. The non-availability of high power pulsed source was probably the reason, why the earlier investigators were unable to get luminescence in case of killer impurities. The ,?ecay co n stan t * b' which is a measure o f tra p distribution within the forbidden gap has been found near unity in case of ZnS doped phosphors indicating thereby the uniform distribution of- traps in this case. The large difference of decay constant from unity in’case of CaS phosphors shows quasi-uniform type of distribution in this case. REFERENCES: q • . 1.- J.T. Hand all & M.K.F. Wilkins,. Pro.c. Roy. Soc., FF' • Aldk, 366 (I9k5a). : ' F- , ’ 2 . R.K. Bube, Phys. Rev., 80, 6 F '<19;'°)« . 3 . v. Lehmann, J. Luminescence, 0/ pw." L-. P. Lenard, F. Schmidt & R. Tomascheck Handb.Exp.Phys„ 23 (Akadem. V erla g sg es., L eipzig, • 5. 0. Schellenberg, Ann. Phys., U. 249 Phve 6 . M. Av-inor, A. Carmi & Z. Weinberger, J. Chem. Phy-., 35, 1973 (1961). f - 180 - Diode la se r Spectroscopy of Isotonic Ammonia I H.D. P atel, H, D'Cunha, A* Jo sh i, T J . Kartha, and V„A. Job Bhabha Atonic Besearch Centre, Trombay Bombay 400 085 The spectrum of the Ammonia Molecule has always been of great importance in many major advances in Quantum Electronics^1"’^ • The infrared spectrum of ammonia in the 10 m region is of particular importance since a variety of laser spectroscopy experiments like Zi\ ■» (. c) double resonancetwo-photon spectroscopy ' % coherent transitions^, Laser,,stark spectroscopy ^, etc,, can be conveniently carried out in this region using lasers like COg laser, s^mi-conductor diode laser and ammonia laser. For the theoretical and experimental molecular spectroscopists, the-i g hand provides a te stin g ground for study of advanced -theory of vibration-rotation spectra, since high resolution spectroscopy of isotopic ammonias can provide enough accurate data for application of any extension of current th eo rie s. Though a large amount of very accurate data is available for ' •jc 5 ____ there is very little data on HH^, 14 and 15 BH^D, BHDg, and HD^. In our laboratories we are carrying out a systematic investigation of the high resolution spectra of these iso^pic ammonias and some of the results are presented in this paper. The 15 NHj sample used* was 57 per cent enriched in KHj D was prepared by equilibrating natural ammonia with D2O of appropriate concentration ad drying the equilibrated sample over metallic eodlus» Spectra - 181 - were run with sample pressures of a fraction of a Torr to a few Torre and path lengths of 16 cm and 2 meters. A laser Analytics LS3 Laser Diode Spectrometer was used for recording the spectra. The spectrometer employe a Snx Te laser mounted on a cold finger and a HgCdTe detector cooled to 77°K. Coarse tuning of the laser frequency was done by varying the temperature and fine tuning by changing the laser current. The signal, without sample, with sample and with a 3" Ge e ta lo n are sto re d in an HP 9825A computer and th en pro cessed and the calculated absorbance or transmittance is plotted using an HP 9872 Plotter. 15 Measurements on about 100 lines of HH^ and several lines of ^NHgD were carried out. The lines were calibrated relative to accurately measured lines of C0o and OCS^®^ using fringes obtained from a 3" Ge e ta lo n (FSR = 0.0163 cm ). 16 A typical spectrum of NH^ is shown in Fig. 1. Repeated measurements have shown that the accuracy in favourable cases is 0.0002 cm""^ and 0.001 cm**^ in less favourable cases. A least square analysis, of these lines has been carried out and molecular parameters upto sextic centrifugal distortion constants have been evaluated for Further work is in progress in evaluating the constants for HHgD, NHDg and ND^ as well as in applying the theory of higher order distortion constants for asymmetric tops and for calculation of anharmon'tc force constants of cubic and quartic orders. i \ r - 182 - NH3 (927.32328) 2.63 0.01631 cm 2.03 UJ 0.83 0.23 SWEEP CURRENT (mA) FIG.1. DIODE LASER SPECTRUM OF 15NH3 AT 927.3cm1 References 1. C.H. Townes and A.L. Schawlow, 'Microwave spectroscopy', McGraw H i l l Inc., (1955). 2. P. Shimizu, J e Chem. Phys. 5J., 2754 (1969). 3. T» Shimizu and T.Oka, Phs. Bev. A, 2(4), 1177 (1970). 4. T. Oka,-J.-Chem. Phys. ^8, 4919 (1968). 5. S.K. Freund aid T.Oka, Phys. Rev. A, 13(6), 2173 (.1976). 6. J.K . Levy, J.H .-S . Wang, S.G. K ukolich and J.I. Steinfeld, Phys. Rev. Letts. 29, 395 (1972). 7. S. Urban, V. Spirko, D. Papousek, R.S. McDowell, N.G. Nersen, S.P. Belov, L.I. Gershstein, A.V. Maslov- skiji, A,P. Krupnov, John Curtis and K. Narahari Rao, J. Mol. Spectrosc. 79, 455 (1980). 8. J.S, Wells, F.R. Petersen and A.G. Maki, Applied Optics, JJ3, 3567 (1979) - 183 - Two Step Excitation Technique for Dissociative Spectroscopy of Na_-Moleculo H.P.Borgaonkar, A.S.Inamdar ,Neela Mehendale Department of Physics, University of Poona, Poona -7» Absorption Spectroscopy has been widely used to obtain spectroscopic information on atoms and mo le cu le s » But the transition to dissociative states have usually led to uncharacteristic absorption continua and, thus, have not resulted in the depth of understanding that has been reached with well bound states of molecular systems ( l ) . I n th e present work, we have reported the two step excita tion technique for the study of the molecular disso ciation processes of sodium molecule using two separately tunable dye lasers pumped by a single nitrogen laser. Ml 1 3 Li DYE CELL PRisn MONOCMttOt'lATOR 6 2 . X -t RECORDER ____ F r $ - i - 184 - The experimental set up mainly consisted of two independently tunable' dye lasers pumped by a single nitrogen laser. The;output of the two dye lasers was focused at the centre of theesodium cell simultaneously. The cell temperature was 550 K. The estimated concentration at this temperature is 10*6/ cm3and a few percent of sodium is present as Na^molecule (2). The fluorescence from the cell was detected at right angle through a prism monochromator by photom ultiplier tube (BMI-98 o 4q B) and recorded, by the Box-Car averager on X-t recorder. The whole set up is illustrated in Pig. 1. Net, Na 6I0 nn-i 8 Irr -it. FIG.-2. The Pig. 2 shows the energy level diagram of molecular sodium. In order to check the presence of molecular sodium and possibility of Naj_excitatlon to different states a laser wavelength was tuned in the range 450 nm to 480 nm using coumarin-1 dye and was made incident ±n( the sodium cell so as to excite molecule from X ^ to B 17Tuor A 1 ^-u near the dissociation of energy dissociating in 3P of sodium a to m . T he 589 nm f l u o r e s c e n c e fro m 3P~*>3S o f a to m ic - 185 - sodium was monitored. It was observed that the 589 nm fluorescence disappears at low incident energies clear ly indicating that the molecular Sodium 1 present in the cell excites to upper molecular states ( 'TTy o v ’jLvt ) and dissociates in atomic sodium. The molecular transition from X to some higher molecular states was achieved by two step excitation ( X l£.at"-> 3 '7]y o-r rt and then i2>'fly «rv-A'^g£to some higher dissociative molecular states) using the laser wavelength Ai ( f or X Q'nw All these excitation and de-excitation processes presented was discussed wittr the help of energy level diagram. As the transition rates of 5S~?3P and 4d—>3P are very much different ( 3)9 the decay of 3P~* 3S when monitored on the time scale showed two distinct peaks at higher energy values of Az-and disappeared above 590 nm indicating that at higher temperature of sodium, molecular sodium can be excited to higher molecular state by two step excitation technique and atomic de—excitation confirms the molecular dissociation in the atomic sodium. References i 1. C.B.Collins. J.A.Anderson et al. Phys. Rev. Lett. 44, 3, PP 139 (21 Jan. 1980). 2. J.P.Woerdman, Opt. Commun, 28,1; pp 69 (Jan. 1979). 3. NSB Hand Book - 22; Vol. 2, - 186 - Two Photon Absorption Technique for the study of De-excitati.on Mechanism in Sodium Atom. A.S.Inamaar , H.P.Borgaonkar, Mrs.Neela Mehendale Department of Phystss, University of Poona, Pune-7. Alkali atoms are attractive for the study of two photon processes because of their simple energy level scheme, atomic vapours can be obtained with- ease, rela tively large two photon absorption cross-section and mainly availability of laser wavelengths for the studies. The present work deals with the 33453 excitation of so dium atom by TP a by moniterir.g the 3P-*3S transition for intensity and time duration in order to study the de-ex- sitation mechanism of the 5S.state. Rate equation were set and their computer solution was in good agreement v/ith the observed decay time of 3P state. Figure snows the schematic diagram of the experime ntal set-up. Dye laser wavelength, after adjusting care fully to 602 nm was focussed at the centre of sodium cell was controlled for the temperature of 180°C. The fluore- sence collected at right angles to the incident beam was focussed on the entrance slit of a prism monochromator. a pnotomultiplier operated in D.G, mode, at the exit slit of tne monochromator in- conjunction with Box-Car-Integra- tor (PAR model 162 main frame and model 164 gated integ rator) ana x-t recorder form the .detecting and recording system . Position of the monochromator was kept fixed at 589.6 nm (3P->3S transition of Na atom) and the decay curve for the fluorescence emitted was observed. The ex perimental decay time was 150 nSec. The decay curve was recorded for different temperatures of the cell ranging from 150°C to 250°C. The decay tim e observed fo r th ese different temperatures was same-for all these temperatu res indicating that the collisions with the excited atom s, probably had no significant effect on the de-excitati on process. Rate equations were set up by taking into account all possible paths taken by the atoms in 53 state to p o p u late 3P. These a re 5S-4 3P, 53 ->4P->3D -#3P and 5S-* 4P-?3S—>35“. The transition rates for all these transiti ons except the 53->4P transition are obtained from NBS Data Book. The transition rate for 5S-*4P is notvwellarc reported in literature. It was calculated by using the mean l i f e o f 53 s ta te as given by K a ise r (1 ), B ates and Damard (2) and Anderson and Z ilits (3), Using the values of the mean life of 53 state, as reported by the above- mentioned authors, corresponding values' of the. transi tion rates for 55 to 4P were calculated and were separa tely used for the computer solutions of the rate - 187 - equations (on ICL 19045). The decay time of level 3P is obtained to be equal to 156nSec, 155nSec and 154nSec corresponding to the three different values of the transition rates for 5S —»• J>P as calculated from the mean life times of 5S state reported in th e l i t e r a t u r e , mentioned above® The observed decay time of level 3P which is 150nSec is in good agreement with the computed values® The observed decay curve shows a small peak at 75nSeo Similar peak is seen in the computed decay curve for. the transition rate equal to 0.0688 x 10s sec*' , obtained from the mean life of 5S state reported by Kaiser (= 7InSec). this favours the' transition rate equal to p . 0 6 8 8 % 10 “3 secto be more accurate. The observed and estimated curves will be discussed® The authors gratefully acknowledger DAE for the financial support of this work. 3> X £ L A S E R r~ o Sc + AMP P R xsn MOMOCHRomHTOR B O y -C A R AVEAftdjER. PAP. MAIN FRAME 1 6 2 . + IN T E G R A T O R . SCHEMATIC DIAGRAM OF EXPERIMENTAL SET-UP - 188 References : ).„D .K aiser; Phy. L e tt. 5IA. 375 (1975) 2.D.R.Bates and A.Damgard; PMil.Trans, £242. 101 (1949) 3 .E.i’i.Anderson and V.A.Zilitis; Opt.And Spectro. 16.99 . . (1964) - 189 - ,' arametrlc Conversion la Resonantly Excited Potassium And Sodium Vapours * B.Bhatnagar Laser Section Bhabha Atomic Besearch Centre Bombay—400065 Stimulated Electronic Baman Scattering (SEBS) and four wave mixing processes in metal vapours have.been a subject of investigation by several authors. Excitation of the medium by multiphotons to generate SEBS on optically forbidden transition was demonstrated by Bokni and Yastiv^ . Interaction of the scattered wave with the pump wave in a four wave mixing process was demonstrated by Lumpkin et ai2^ SEBS on a number of cascading transitions in Caesium vapour was reported by Sorokin arid Lankard^^. In this communication we report SEBS on several cascading transitions from potassium and sodium vapours. The experimental arrangement used in these studies is shown in fig.1. Metal vapours were produced in a heat pipe oven, of 15 cm zond length (pressure 10-15 Torr). The output of an Excimer pumped dye laser (0.6 MW) was focussed in the zone by a short focal length lens. The emission was collected by another lens and focussed on the entrance slit of a 0.5 Crzney-Truner spectrometer (Spex 1702). The spectra was recorded using sample and hold integrator circuits and strip chart recorder. The potassium vapours (350°C) were excited to K(6P) and K(5P) states by 344 nm and 405 nm output from the dye laser. With 344 nm excitation emission occurs at 388.6 nm, 405 nm, 412 nm and 769.7 nm. (Pig.2). With 405 nm excitation the emission^at 465 nm, 544 nm, 569 nm, 576 nm, 779.6 nm, 801 nm, 1152 and 1172 nm.. The origin of this emission can be explained as due to SEBS and subsequent mixing of the scattered wave with the pump radiation via the processes63 pump-2 *-*3 scatter and t*> pump- (^scatter 1- ^0 scatter 2. The calculated wavelengths for various transitions along with the possible processes are listed in the table. Good agreement was obtained with the experi mentally observed wavelengths. Similar emission was observed when sodium vapours (Temperature 550°C) were excited by 330 nm output of - 190 - the dye laser (See table). It was observed that the wavelengths resulting from four wave mixing process specially those involving lowest resonance states e were emitted in a fairly large forward cone. The spatial spread be due to the fact that, in a four wave mixing process non-colinear phase matching is possible at large angles if one of the wave invol- (4 ) ved is generated internally . With two of the participating waves generated internally phase matching may be possible at even larger., angles thus attributing to large forward cone emission. References; 1. M.Rokni and S.Yatsiv: IEEE J. Quant. Electron. QE-3, 329 (1967) 2. Q.J.Lumpkin, Jr.,P.P.Sorokin and J.R.larikard: Bull Amer. Soc. 12, 1054 (196?) 3. P.P.Sorokin and J.R.lankard: IEEE J.Quant. Electron. QE-9, 227 (1973) 4. A.Corney; J .Phys.B Atom.Mol.Phys.:12,1425 (1979). * The work performed at University of Hannover, F.R.G. Pi B K C tMBA LASER /f£ A T P (? e OVE/V MONOCHROMATOR. CHARI R&C0A6 8 1 SAHPUS-HOLO in t e g r a t o r . ch.t . - 191 - FIG2 £xcitatio*i tdAvEL£K/gW 3 4 -^ '7'»<» PoTAss/urf 4 - ° 5 f 388- <£ ExciTfiiTiotJ MAveuZfi/STh . 6 s WAVELENaTh CNfhj V6ATICAL SC ALP tw TABLE 1 - MATERIAL, EMISSION PBOCESS BXCITATIOB >;J WAVELENGTH WAVEI33GTH Io ucn t,'CO ito I a §► z**X I CALCULATED (w*t) B (ran) POTASSIUM 344.7 (4S-»6P) 388.6 387.68 “ (6p )-2 w (6p .*4d ) 405.55 404.84 ^ ( 6 P)-L*l ( 6 P->6 S)- 0 ®(6 S^6 P) 405.85 404.825 <^(6P)-^ (6S-»4D)-U)(4IX#5P) 769.7 770.19 W (6P)-u)(6P-»5S)-ul(5S-*4P); 404.88(4S-#5P) 544.65 544.45 (5P)-2^(5P^5S) 576.45 576.335 W (5P)-2^ (5P-#3D) 1152.7 1152.4 D (5 P )-2 u )( 5 s-#4P) 560.0 559.94 03 (5P)-ti(5B^ 3D)-U) (5P-»5S) 769.6 770.7 °(5P)-U (5P-je>D)-u)(3D->4P) j 801.0 801.47 ^ (5P )-ti( 5P-* 53 ) 3D -»4P) 1172.8 1169.7 (3D->4P) 0PS6 465.25 464.38 V) (3 D ^ 4 S) 0PS£ SODIUM 334.7(3S-»4P). 462.7 462.95 U(4P)-20J(4P-»4S) 593.2 599.9 U) (4P)-U)(4P-»4S)-C^(4S -»3P) 3o L 7—1 P > ___ 6 4 , EKJ6 R61V LOVft S~— POTAtr/VAl 3 a* _ 5___ 4 10 4 193 - Laser Induced Intensity Changes in Atomic Lines of Uranium Excited in a Hollow Cathode G.D. Saksena and A.K.Tripathy Spectroscopy Group Bhahha Atomic Research Centre Trombay, Bombay-400085 Resonance absorption of laser photons by atomic specie present in a hollow cathode discharge tube signi ficantly alters the population distribution in the different excited states of atoms resulting in marked intensity changes in their atomic spectra. We have observed laser induced intensity changes for a number of neutral uranium atomic lines emitted in a hollow, cathode lamp, when the discharge in the cathode was irradiated with laser radiation corresponding to the 5915 A reso nance line of uranium. A hollow cathode lamp of standard design was fabri cated in tfie laboratory. The cathode consisted of a hollow cylinder machined from uranium metal. The anode was a ring made out of tungsten wire. The lamp was filled with neon at 5 torr, and was operated at discharge currents varying from 50 to 150 mA. Intensities of the . hollow cathode emission lines were recorded using the i THR 150Q high resolution scanning monochromator. Resonance absorption of the C.W. dye laser radiation by the 5915 A uranium line was constantly monitored by the opto-galvanic signal. Two lock-in-amplifiers were used, one to monitor the opto-galvanic signal and the other to record intensities of the spectral lines. The experimental arrangements is shown schematically in F i g . l . Intensity changes were observed in 31 UI transitions c o v e rin g i region 6129 A to 5813 A,(the rhodaine 6G dye region). The transitions involve a number of the known low odd levels (upto 10,347 cm”^) as the lower levels. Almost all the transitions show intensity changes implying that a general redistribution of atomic popula tions in the excited states of uranium takes place. The UI line 6056.80 A (0-16,506 cm”^) shows the largest intensity change. This is obviously due to the fact : that the 16,9000 cm”^ level is strongly coupled to it. The relative changes in the population of various levels w ill depend how they are coupled to one another. CHOPPER LENS LENS DRIVE 0 Y E ♦ 0-5 KJV FJG.1* SCHEMATIC SET-UP FOR STUDYING LASER INDUCED CHANGES IN URANIUM HOLLOW CATHODE SPECTRUM BY 0PT0GALV0NIC DETECTION. - 195 - Resonant two-photon (two-step) ionisation of uranium atoms has been recently reported in literature by irradiating uranium vapour produced in an oven, with the UV lines from an argon ion laser. This is explained on the basis that the 3345 A and 3511 A groups of Ar+ laser lines coincide with the uranium transitions 3344.74 A (4276 - 34,165 cm'1) and 3511.14 A (4453 - 32,926 cm'1) respectively and therefore uranium -1 atoms in the 4276 and. 4453 cm meta-stable levels are ionized by two-step photoionization. We have irradiated the uranium hollow cathode discharge by the UV lines from an Ar+ laser and confirmed ionization of uranium atom s. The experimental technique was sim ilar to that described above. Using a quartz prism, the three groups of lines around 3345, 3512 and 3638 A were separated. The ionization of uranium atoms was detected by enhance ment in the intensities of the uranium ion spectral lines* The results are shown in Table I. It is seen from the. table that when the discharge is irradiated with the laser lines at 3511 A and 3514 A, a large increase in intensity,z^I/I=48.3 , is observed in the 3879.53 A UI line. This is obviously due to a large depletion of atoms in the 4453 cm"1 level due to absorption by the 3511 A laser line. This is further - 196 - verified indirectly by the negligible intensity change in the 4186.96 A UI line, which does not end in the 4453 cm-* level. The increase (4%) in intensity of the 3860 A U II line is direct evidence of an increase in i the U+ ions in the hollow cathode discharge on irradia tion with the 3511 A line. T a b le I UV Laser Induced Changes in UI and UII Lines Emitted In a Hollow Cathode Discharge Lamp Classification of °f U Lin© irradiating intGnsity in U 11H6 l a s e r l i n e 3879.53 A UI 3638 A N e g lig ib le (4 4 53-30222 cm- 1 ) 3511 and 3514 A + 4 8 .3 4186.96 A UI 3638 A N e g lig ib le (4276-28153 cm- 1 ) 3511 and 3514 A + 1 .0 3859.58 A UII 3638 A + 1 .0 (289 - 26191 cm-1) 3511 and 3514 A + 4 .0 I LASER SCATTERING Laser scattering by nematic liquid crystal under very low frequency ( <.1.0 K_) excitation - S.Jatar, P.I< .Puntanbekar and V.l.Bhide, Effect of laser field on compton profiles - B.S.Shaima, A . ’ T .Iripathl and 3.5.Singh. First-Order Space-Time Correlation measurements of a Laser Beam Propagating in a fluctuating medium. - Budi Santoso aid 'foshihiro Ohtsulca. H ioton correlation spectroscopy - Some Industrial Applications — S.B.Dev. - 197 - LASER SCATTERING BY NEMATIC LIQUlft) Crystal under very lo U frequency "?*" Hz) EXCITATION-1 S. Jatar, P.M. Puntambekar and U.G. Bhide National Physical Laboratory, New Delhi.110012 Nematic liquid crystals when subjected to an electric field show several important effects. Since the first observation of field induced in stab ility by Williams []1 3 a large amount of uork has been done to study these instabi lities in the high frequency range. However little attention has been paid to study the effect of very low frequency (-4 1.0 Hz) a.c. field on a nematic with negative d ielectric anisotropy. Present paper reports the results of a nematic material under low frequency excitation.through the study of transmitted intensity of a He - Ne laser beam. The experimental setup consists of a homogeneously aligned nematic liquid crystal cell along with polarizer and analyser which can be oriented with respect to the unper turbed optic axis (UOA) of the nematic.. The liquid crystal cell is made by sandwiching nematic material E8BA between two transparant conducting glass plates with the help of a 12 yum thick mylar spacer. The homogeneous alignment is obtained with the help of obiquely evaporated silicon monoxide film . A He - Ne laser (power 2 mW) is mads incident on the cell after passing through the polarizer. The - 198 - transmitted intensity of this laser after passing through the analyser is made incident on a photomultiplier tube. The out put of this defector is fed to the Y-axis of an X-Y recorder. The cell is excited by sinusoidal wave with amplitude 20 volts and frequency in the range of 0.001 Hz to 1.0 Hz. This applied voltage is also fed to the X-axis of the X-Y recorder. At zero applied voltage the LC cell shoua maximum transmission. Uhen subjected to a.c. field, the cell passes through various instabilities and finally shows dynamic scattering above a threshold voltage. For very low frequencies ( 4 0.003 Hz), the sample completly follows the a .c. field but as the frequency increases the sample shows some time lag leading to hysteresis. The nature of the hysteresis loop depends upon the orientation of the polarizer and analyser with respect to the U0A. In one case when the polarizer and analyser both are parallel to the U0A, the hysteresis loop extends- in the dynamic scattering range whereas in the other case when the polarizer and analyser both are perpendicular to the U0A the curve shows pronounced saturation in the dynamic scattering range. The area under the hysteresis curve is seen to depend upon the frequency of the applied fie ld . At frequencies above 1 Hz a sta tic picture of in sta b ility is observed which depends upon the magnitude of the applied voltage as formulated by Dubois- V iolette at e l £5^. - 199 - This phenomenon can be explained on the basis of the change in molecular orientation caused by application of fie ld . In case of a homogeneously aligned nematic with negative dielectric anisotropy the moleculee are aligned along the plane of the glass plate whereas their dipole moment makes a finite angle (<90°) with this plane... When ®n electric field ia applied the molecules experience two opposing forces one due to the applied field trying to align the dipole moment in its direction and the other due to the surface aligning forces which try to keep the moleculee in their original homogeneously aligned position. Above a threshold voltage the force due to the field exceeds the force due to the surface alignment and an instability is set up. For a sinusoidal field the molecules oscillate between the two positions the.unperturbed homogeneously aligned position and the instability position and the rate of this oscillation depends upon the frequency of the applied field. For very low frequency the molecules follow the field oscillations and no hysteresis is observed but as the frequency is increased a time lag between field oscillation s and molecular o s c illa tions is produced which gives rise to hysteresis effects. This time lag ia reflected in the transmitted intensity of the laser beam. At frequencies above 1 Hz the molecular dipoles do not get sufficient time to relax to the equili brium and hence .show a static picture of instability corresponding to low frequency a.c. regime. - 200 - R eferences 1 R. U illia m s , 3 . Cham. Phys. 1965, 39, 384. 2 R. W illiam s and G. H eilraeier, 3 . Chem. p h y s. 1966, 44, 638, 3 3.0. Kessler, M. Longley and U.O. RaSmussen, no 1. Cryst. Liquid Cryst. 1969, 8_, 327 . 4 L.K. Vistin, Sow. Phys.- Cryst. 1972, 17^, 842.. 5 £. Dubois-Violette, P.G. de Gennea and 0. Parodi, 3.de Phys. 1971, 32^, 305. 201 - EFFECT OF LASER FIELD ON COMPTON PROFILES B.S. Sharma and A.N. Tripathi Department of Physics, University of Roorkee, Roorkee and G .S . S in g h Department of Physics and Astrophysics, University of Delhi, D e lh i The strong electric field of the coherent sources modify the electronic states of a system and hence any physical quantity •which depends on the electronic states would generally get modified in the presence of the laser field. The present work is concerned with the theoretical investigation of the suitable conditions pertaining to the experiments which would measure the modulation of the Comp ton profile for X-rays Coapton-scattered from atomic systems placedLin the laser cavity. We further examine the situa tion when modulating part would reveal structures and would help in exactly locating the points of inflexion in the unmodulated profile and the positions of nodes in the momentum-space wave functions. The laser-field modulated X-ray Compton profile for an atomic electron of nlth orbital may be written in the f o r n r t 2 : , Y __L . S x * L - A - * * ® J n A ) = '*•(!) Here J n£(q) is the usual Compton profile2 and x2=e2E2/'fc2 E J ( 0 ) J L (0 ) L i 3 .0 2 .5 9 2 7 2 .3 6 9 1 —8e 60 Be 4 .0 3 .1 5 8 8 2.8810 -8 .7 7 B 6 .0 2.9895 2.7215 - 8 .9 6 C 8 .0 2 .8 7 7 1 2 .6 2 1 9 -8 .9 0 N 8 .0 2 .7 9 8 6 2 .6392 —5.6 8 0 8 .0 2 .7 7 6 2 2 .6 7 0 2 - 3 .3 2 F 8 .0 2 .7 5 1 4 2 .6 7 7 6 -2 .6 9 Ne 8 .0 2 .7 2 7 6 2 .6 7 4 3 - 1 .9 6 2 We further consider the situation where x Is so small that contribution comes only from upto second term in Bq.(l) and hence the,,modulation Is proportional to the second derivative Jni(q) of the Compton profile. More over has its importance due to an experiment sugges- cea by'"Jala and "soar1 for modulation studies. Hence Jnt(qJ and J^^q) for occuPisd orbitals (Is and 2s) of atomic lithium are plotted in the following figure. 3 2 1 -1 -Z -3 4 -S 1*6 ' 2 o - 203 - We observe that the derivatives exhibit wide variations and structures than the corresponding Jn»(q). Upward and downward crossings of the q-axls by any J'n£,(q) curve correspond respectively to the points of inflexion in Jnj_(q) and the position of rtode (qQ) in the romentum-space wavefunctlon of the nlth orbital. The value q0=0.93 for 2s orbital is in close agreement with that calculated by Cox ® The data given in the table reveal that for lighter elements a greater decrease in height of the profile at q=0 Is attained at a relatively lower laser field and hence in view of condition (ii) low-frequency lasers would- be more favourable for performing such experiments. Fur thermore the lighter elements would be more suitable for measuring laser-induced changes and this is in conformity with the findings of Ehlotzky.7 R e fe re n c e s 1. M. Jain and N. Tzoar, Phys. Rev. A18T 538 (1978). 2. B.S. Sharma, U.S. Singh and A.N. Trlpatbl, to-be published. 3. B. Williams, Compton Scattering (KcGraw H ill, New Y ork, 19 7 7 ). 4. B.S. Sharma and A.N. Tripathi, J. Phys. B 1 2 . 3 1 0 7 (1 9 7 9 ). 5. E. Clerontl and C. Roethi, Atom Data and Nucl. Data Tables £4, 177 (1975). 6. H.L. Cox Jr., Phys. Rev. Alff, 229 (1976). 7. B. Ehlotzky, Phys. Lett. 69, 24 (1978). 204 - ^ a^-co v i^ y ue/y ^ ts (S'J CL b i s C C \ ^ ' t'frv- ^ *iuxJLL JcrnXko Ma£omn£ (Hmvvie Cw-WaJ* ^t>dx,‘'$\s£'Le I* 6^ K dll*^L> AL*\CwuiL, io-fyp^no 7 m ^ . Ejt ' i l l uLku ^ ^»v€<4^Vvu^, e<*'l/lc6«u£tXW ** $-i-&a^oc*£ tPwo ^tvn^eftox. 6^/WtA* V *-(r+ \^ -v,--# ,• . - 4-_#. 2P., -«,VL., _ - 1 - 4 « I » . „ " U V - t J V J IVtTD-vw 'MUL, ^ W t iw A ^ 'H M U l . ^ •u VM7-U^{v ~&JU £wVU- A*<*x>*v - ^ktze-tix^V^um/# 6^ £»»V^<*t«CoU' "Uv^/vo vyv rVIX-cntx 'fituZ*L ■&■£ mXt^Mt^vvv^ *A-t. *»'^vv4v^. ~ t»Vve&v(v ‘tiv£ -6<1* PiJUa^ £*/lVvdU& #w £- T-f?6 |»-ttA^»-tC, ^-c<» ^6&*\SVV^- &**£ *rt-VnMAAAs*n&v$a. ^ K. ^ z iO ^ ^ O z * ~ 205 - Mxe. vpAyzsL tvM-ue- SguVue&^tJvv ^vr'^Z vyv a_ ^UV&V * ■^'*1 "^vy 'CKjL&ti.*¥t£*yX "tit£ ^fL*^fcceL&/ovs >n £ S&£-/Up -Li S^Lewrv ^*v» $*£■' I • w^ ^ c - 4 . ^ ^ t t x — Trptsqh& vtytijtA* a -£^ry-^p-^JL uA/C^ 5*r*> XtlVv'fx-;^.. w-c- >vv*1t-*<£^X#v<. fxit^ce-T^-ecM "tJ^e^-e **a4vJhJZ&r$ cvu. -fly OC^J. -^1L Oz^fJLeA-Zrdlj . 4^a. ^*. Mcr/J-e. -leiA-tA. fp-^yuut^txt^ tvv tULe^ Kc- -t< l&\- c j 0^ ^2.^ o ^ iX wi-- u)0+ .fl^ •«)„ ^-v>^ ^ '^tA, . T&< u>| XZ3 /jl ^ l JL t>W CL'Vfc^6-v«^v*C "^6 e *>x/ ^ iX/i^vc^iL £t cf£ud L l_ vTKv-C. f-€«u£e. (6( P) O - f ^ . T-^t>UcJv*Ct_ $1jl-S^U f-jp "tt\jL "ft-t/vv Cclwv^* iVVV^ft,^ CLe e * A 5< **f{*L ^t-i-s)}+ p e ^ («i,t+ %.t?)j t s j X" 4*vJL>^ «a< yurv>~*J. V U & V ) di^xa^l^g^c^ tt^pS-Vyfc^ C^'PcCXlS - "Hvfc <-^L ^ e w v ^LM *4. $vt-*->*plj&4. c t^ v ® 67*^jt/ve vk**tC ecL - -C^eeZLve ‘ff^t£d- c* zv v 't^ 6/»-u &<- iufXiMi^ cto ? * ^6,t) *? S 4 ^ 4 ‘Vj A ^£ic/v, SovvC ^ D , *-» I x = ^ A-C0 , t ) *<>•* L&tiit+$&£)+ cj 0 • V J* - ** «*• [*<**■*- fy* K^Xs.t) ^^><$5 - 206 - Ah>-u>,-lJv CMA- C L ffU * S t* U ■ "& t x X~U .ty £wvvty ji C Ctxt. Ay OfM^kny. i l ^ fix's'fij }*&yti*AX -vJ^..o i^)c ery AU**- tlCTVV^ ^V€zd^_ ^wxzvft^v^a C*s W t ^ . t>nz<*>M/v»v£'&- eKtr\A*^cJiL Z^Locg fix (t.t)-a/< rx (?,t) J* t iM ) > e <4. (o,t) A Ci,t+r) e^[iu)Lt^(p.t> fk-t+fyp-rl> t %Li ,r) « ai < I x(o,t) J>^ >ttr)> - <4L0,t)A(t)t+r) s^u[^iorf ^ ^ t)-^ ,t+r)4p^> & U -.'Sc lAvte © Cy f X 5^ < a ( o , t ) 4 jr* JtvLa/- Ac. l?v»/Wt»vE;2t ^ Sfl^l6p w * t ft ,r) -e x f [ < 6R.C)) n * t t Kt&.t) « \l cJ,'-te>v ^ ■ 6k,t)* tv (4,/e*)-«.»r fWa-vdbta ovte. S^tnvK , Vn- -^Uj. 2- CVvv-^- 3 Fig.| Oio^rom o| twc experimental ee* up 1 C* ,7) Fig. I m-i . Q m m ftS-beevn gpViiur 1.0 (Vi. mirror O'deUctor a. 2 mm q p . quarter WQVt plate •a <| m m ULM-l pM. jfcuetoatmg medium p - etcttronie *lusr € _ Correlator R-ueordef 5 - 05etito*cop« > 4 t/i* - Ultrasonic. . rnodvtator, BE- bcowi e t p a n d tr r msec P - PoL*ri*er 18 Re>erenec9; t.v.et$o»9io, CoMrtnW Optitoi §ft)*nur>i^ edited b # F.j. flrr&fiht . oti<* v otgior^ie.H H t-Ian, pp,8t - 195. 2.8 •ttro&iyneni, p.Oi P»rto, m • Bcrtoitoti .statistical Prer»yti»t 207 ®i Seatterod light, ftp.ieu . 3 w aorn ^ 6 Uet## principle* Optics, Atrgomon Pre*5 *MS. PrjoO ^,y.O*t6u**, 3-Jovr. flppl.Phf* .vbt n.vtpf, 197$, pP#37?-u9l 5 . R .t. F a n tc , P ro c r 66G , Uoi. 63 >i'0 « l t «315 PP f 69 3 » I b ^ t f.R-P.U ltom ir**^ pr»e So* pHebo Op* Inftir 6«9>«U. .|3 - 0,6 ' I 87i, PP * *■* -6 ). fl.Soemarjono and H.Hirono, P ret ■ 9l Photon Correlation Spectroscopy - Some Industrial Applications S.B. DEV Hindustan Lever Research Centre Chakala, Andheri (E) Bombay 400 099 In this short paper, I am going to present some preliminary results we have obtained on the measurement of diffusion coefficient, hydrodynamic radius and polydispersity on gold sol, polystyrene latices and gelatin samples byPhotoraCorrelation Spectroscopy (PCS) - sometimes also known as Intensity Fluctuation Spectroscopy (IFS). Recently we have bought a 48 channel Malvern photon Correlator fitted with a dedicated HP 9815A computer and various other accessories. This spectrometer will be used in some basic studies on polymer conformation, micellar dynamics, adsorbed layer thickness, sub-micron particle size distribution and a host of other applications. The set-up, shown in fig.1, basically consists of (i) a spectra physics 15 mTT He-Ne laser, (ii) spectrometer and optical specimen cell, (iii) photon detection system which has an EMI photomultiplier tube of low correlation and dark current, and (iv) a digital correlator. There is also provision for measurement at different scattering angles, made possible by continuous goniometer rotation from 0 to 150 degrees. Temperature can be controlled within 0.05 deg C in the range of 10 - 150 degs. C. For the low and high temperature in this range, an external recirculating system is used. For the details of the PCS technique - theoretical and experiments^ ^ the reader is referred to two excellent volumes that have appeared ’ on the subject. Basically, PCS analyzes laser light scattered by a small volume of dilute solution of particles undergoing Brownian motion. The rate at which the photon arrives at the detector is linked with changes in the molecular position caused by random Brownian motion. The Malvern system employs advanced signal processing technique and can follow departure from randomness in the arrival times of these photons. It is obvious that the laser frequency is several orders of magnitude higher than the frequency of the random Brownian motion. - 209 - What the correlator achieves is to extract information about this motion which can be related to various molecular properties. The random Brownian motion of the diffusing particles modulates the scattered light intensity and causes broadening of the laser linewidth. It is possible to calculate the particle size from a knowledge of the spectral breadth. Of course, as in conventional light scattering, results can be obtained for molecular weight, second virial coefficient and radius of gyration of the molecule by investigating the variation of the scattered intensity with angle and concentration. If the sedimentation coefficient is known, however, then combining this value with diffusion coefficient, measured by PCS, yields an absolute value for the molecular weight. The technique of PCS is far more powerful than the slow and cumbersome method'of using a spectrum analyzer. One of the most important functions in Correlation Spectroscopy is the so-called auto-correlation function (ACF) of the scattered light frcm which diffusion coefficient (D) is computed. The intensity of the scattered light l(t) would fluctuate when random Brownian motion is superimposed onit. One could get a measure of the typical fluctuation time tc , also called the coherence time, from the ACF. The value of l( t +T-), at time t *"L , is in a way conditional on the value of l(t). If the delayX. is large l(t) and l(t +T.) are independent of each other whereas for short T., the dependence is maximum - so that ^l(t) l(t tt-)) = Al 2 at z_= 0 but for large T. » ^l(*) + T ) > = 4y . In an actual experiment, the intensity is not directly measured but information is sought from the analysis of photocurrent which appears as a train of pulses. It can be shown that the experimental parameter which is of interest is nothing but the normalised ACF, viz. t Z. -* - 210 - In the simplest ease, g( "C-) is a simple exponential, so that |jfc)| = (,) 2 where f~ = DK- , D = Diffusion Coefficient, f = Decay rate K '= Scattering vector = ttirn/x (2) n = Refractive index of the solution A.= Laser wavelength 6 = Scattering angle All our measurements were made at 0 = 90° The ACF contains information about the distribution of diffusion coefficients, in the sample - strictly scattering volume. The plot of In of the ACF vs x. , sample time, gives D. Once D is known, the hydrodynamic radius can be obtained, assuming non-interacting Brownian particles moving through homogenous solvent of Newtonian viscosity!^ , from the relationship D = kr/f, where k is the Boltzman constant, T, the absolute temperature and f, the frictional coefficient. If the particles are spheres, then 4 5 6 i r q ^ (3) and hence, R^, = kT/6ltf{D (4) giving the value for hydrodynamic radius. In practice, the logarithmic plot does not give a straight line indicating polydispersity owing to particle size distribution. ' The ACF, in such a case, becomes sums of exponentials, so that ( 5) where G{f") is the normalised distribution of decay rates. . Distribution of D's is quite common in macromolecular diffusion. In theory, G(_n) can be obtained by Inverse Laplace Transform but this requires'tiata of very high precision which is not possible in practical case. Shat is normally done, therefore, is to assume a r , and expand exp (-PC.) in a power series, in other words I e ' c t ) I - ^ K ' r l ) [ityM7i! - i ! 4 . - 3 (6 ) Jkx.iU-'-are the moments about the mean G(r ) - 211 - \ Finally WC<>V)* vU c-rt-* i ^ T V- -- (7). \x Thus, from a polynomial f it one can .getykjpi ih?Leh prorides a measure , of polydispersity. In general, the computer program used in this work treats all sources of polydispersity irre.specttre of their origin. The inherent assumption in the model of the diffusion process is that these are uniform, non-interacting spherical particles. After optimization of the equipment with" regard to clipping level, sample tine, aperture setting on the ESST, samples sere run at Q » 90* a t 25 deg C. 'C.for all the samples was Chosen at 100 ns, the total sample number ID . Several runs sere made for each sample. The following is the av«age iff four readings. 8 2 5 Sample She 10 (mm /S ee) fi^XlO (Cm) P o ly d is p e rs ity Colloidal gold = " 2.12 0.92 0.299 Polystyrene . ' 1.86 la te x 1.14 0.093 g e la tin 1.77 1 . 2 0 ' 0. -*3 For lack of space,.discussion of interacting particles, structure factor* calculation, concentration effect and dynamics of many- partlcle motions is left out. The application of PCS is far too numerous to mention here. The biggest challenge, it seems to me, is to be able to produce a coherent treatment of high polydispersity for nacr omoleculea. References; (1) Photon Correlation and light beating spectroscopy-(Ed) H.Z. Cummins and E.B. P ik e , Plenum, N.Y. 1974. (2) photon Correlation Spectroscopy and velocimetry - (Ed) H.Z. Cummins and E.B. Pike, Plenum, N.T. 1977. Photon Correlation Spectrometer Pig. 1 1ASEH PHODUCED PLASMAS Interaction of Dipole electric field with an electron plasma - S .Kristian and M .Gopalaswamy, Inverse Eremsstrahlung absorption of m u lti-la s e r beams in a Magnetoplasma - U.Gopalaswamy. Effect of radiation losses on scaling laws in laser produced plasmas - S.K.Ooel, P.D.Gupta and D.D.Bhawalkar. lionsaturation behaviour of V-I charac teristics of a plane Langmuir probe in streaming laser produced plasmas - S.K.Goel, P.D.GUpta and D.D.Bhawalkar. Temperature diagnostics of streaming plasmas produced by L asers - G.K.Goel, P.B.Gupta and D.D.Bhawalkar. Determination of electron temperature from ion expansion energies in Laser produced plasmas - P.D,Gupta, 5.K.Goel and D.D.Bhawalkar. - 213 - INTERACTION OF DIPOLE ELECTRIC FIELD WITH AN ELECTRON PLASMA S.KRISHAN and N. GOPALSWAMY Department of Physics Indian Institute of Science Bangalore - 560 012. Over a period of the last fifteen years it has been considered an established fact that a dipole electric field does not interact with a collisionlesq, non- relativistic plasma provided no other external fields are present, and that only one species plays a dominant role. This is claimed to be an exact result. In these(l-3) Investigations and in the subsequent ones, an Infinite by infinite determinamt set equal to zero to obtain the dispersion relation is shown to reduce to a dispersion relation which corresponds to the no field case. The fact that an infinite by infinite determinant is redu-'ed to a simple relation (S) has been possible because the Bessel functions occurring in the expression for the fluctuations in the charge carriers are inde pendent of particle velocities. If somehow the Bessel functions are made to depend on the particle velocities then the above simplification becomes impossible to accomplish and as a result, the dipole electric field interacts with a simgle species plasma. This fact is borne out when one studies the interaction of the dipole field with a plasma in the relativistic lim it and/or under the influence of a constant magnetic field (4). In all the previous investigations on the interaction of the plasma with an oscillating electric field, in the dipole approximation ensemble averaging has been done incoreectly to simplify the algebra. In principle one solves the Vlasov equation taking - 214 - Fourier-Laplace representation of the perturbing electric field and only then the velocity integration is performed to obtain the perturbation in the charge carriers, if for some reasons of mathematical convenience velocity integration is performed to obtain the perturbations initially at some intermediate; stage of the calculations, it is obvious that in the subsequent mathematical steps, time integration is going to be performed in the 'mean pic-bare' whereas, it should have been performed in the 'Kinetic picture' or the microscopic picture, if the time integration is done in the kinetic picture before doing the velocity integration, we get a new dispersion relation involving Bessel functions that depend on the particle velocities. Consequently, a single species plasma with ions in the background interact with the dipole field. A triplet of waves, C0p , + o->|,-CtJo propagating along the electric field of the dipole is excited in a certain density domain near the critical density, where i s the pl&sma frequency and co0 is the f ie l d frequency. The in s t a b ilit y i s due to n egative Landau damping. The dipole field causes the electrons to have a stream ing motion. These streams are Landau damped and the energy goes into the waves. REFERENCES: (1) Dubois, D.F. and Goldman, M.V. ( 1 9 6 7 ) , P h y siv s Rev. L e t t . , .12, 1105 ( 2 ) Ivanov, A.A. ( 1972 ) Reviews of Plasma Physics, 6, 181, Ed,! Leontovich, M .A., Consultants Bureau, New fork. (3) Jackson, E.A* ( 1967 ), Phys. Rev. 153 . 235. (4 ) Paverman, V .S ., Svimonishvili, I., and TSchakaya, D.D. (1980), Physica Script a, 147. - 21)5 - INVBRSBBRBMSSTRAHLUNG' ABSORPTION OF MULTI-LASER BEAMS IN A MAGNETO- PLASMA. N. GOPALSWAMI Department of Phyelce Indian Institute of Science Bangalore-5600 12. The interaction of laser beams with, plasma in the presence of magnetic fields is of wide interest in Laser fusion studies. Multiphoton-inverse bremsstrahlung mechanism of absorption is quite efficient at low temperature plasmas [1,2]. Seely [2] has found that the inverse-bremsstrahlung absorption rate and multiphoton threshold of a single laser beam in a magneto plasma decreases in the cyclotron resonance region. It has been shown by us. in a previous paper [3] that this reduction could be avoided by passing two laser beams into the plasma under certain conditions satisfied by the frequency and field strength of the laser beams. We have extended this work to a case of 2N laser beams of closely placed frequencies in such a way that there are N lasers with frequencies just above the cyclotron frequencyc • \ y y ‘ 4 r - No. Oj- f > ^ ” s °f r* B*1er f -9- 1 * The transition probability for the electron to make a transition from a levelft> to a level(£) is calculated. From this, the change in no. of electrons in the state 1 2^ is calculated by writing the kinetic equation. By taking classical lim it, the change in electron distributionj^(i/J is obtained as, - 216 - ■tuw ~ zv ^ V* Si^-f ) 2)-t W-N v"' T ce- and the change in electron kinetic energy as c L«tt < ^ > - fdV J 1 z at - Trxgxe4N^g aNrz lN iiiii 1 where L-*--, u>r~(*JCt -* Ne,-le|& m are density, charge and mass of electrons, N l is ion density *. cor A B.r dre frequency and field strength of r laser, toC4‘»s electron cyclotron frequency. The e f f e c tiv e c o llis io n frequency is ' ^ej-j v „ 3 \ / — 7^** - 3T^ _ - ^ z.fNi - ' e Nfciei^ —t 3 3 a t n QZ:t-TI, — Cvv-t^cc — TJ where< €.> is average electron oscillatory energy given by / , . _ lei^r v _Br_ I 2" ° 3-W L r=l ^ r 'Wce J From the expression for 'Vefjf. it is obvious that the vc«. and hence the absorption rate decreases as '?Lce and as Nincreases. . But notice that we can . adjust cv’s, above and below the cyclotron frequency such t h a t — . - 1c (£< & r ^ o r H ' w ce Then ■>%- shoots up and consequently the absorption also increases. Physically speaking, electrons rotating in response to each of the laser effectively has a reduced kinetic energy and encounters more number of collisions with ions leading to more absorption. REFERENCES [l] - Seely, J.F and Harris, E.G. (1973) Phys.Rev. A£ , 1064. Seely, J.F. (1974) Phys.Rev. A10. 1863. HI Gopalswamy,N and Krishan,V (1980) to be published in Astrophysics and Space Science Jo u rn a l. - 2 1 ? - Effect of Radiation Losses on Scaling Laws In Laser Produced Plaamna S.K.Goel, P.D.Supta and D.D.Bhawalkar Laser Section Bhabha Atomic Be search Centre Bombay-400065 Study of moderate Z plasmas created by lasers in the flux range 11 13 2 19 to 10 W/cm are becoming increasingly important for efficient hydrodynamic coupling of laser energy to the target. Scalings of 2 plasma parameters such as electron temperature , ion expansion energy^ over laser flux, are usually taken as a convenient method of understanding the various processes involved in this coupling. In 4 this flux range when self regulatory model is valid, temperature (le) scaling over laser flux ( 4>) as Tg~ has been obtained2. This scaling assumes that the energy lost as radiation t s om the plasma is insignificantly small compared to the laser energy ased in heating which may not always be the case. Since experimental scaling laws are determined by relating plasma temperature with the incident laser flux ( and not with the flux used for heating alone), 4 /9 these scalings may be different from Te ~ <£> . In the flux range where the energy loss in radiation is significant, we obtain two new temperature scalings in carbon plasma and verify them experimentally using a NdsGlass laser. The exact temperature scaling will depend on the variation of the relative magnitude of the laser energy lost in radiation to the energy utilized in heating of the plasma as a function of incident . laser flux. Since moderate Z plasmas created by lasers operating in relatively low flux range are usually not completely ionized, radia tion coming out of these plasmas will constitute bremsstrahlung, :recombination as well as line radiation. This power loss due to 2 radiation can be calculated using formulae given by Puell for a particular temperature and their total sum (Pg) can be compared with the laser power (P^) required to generate plasma of the same tempera ture in the case when all of its energy would have been used up in heating. Hatio of P^ to P.^ is shown as a function of plasma tempera ture in fig.1 for carbon plasma. Since some energy is lost in radiation from the plasma during the laser pulse, laser power - 218 - required to generate the same plasma would now be P£ + instead of P^ only and experimental sealing will depend upon how T scales with P^ + P-jV Under the assumption P^^< P^, plasma parameter T^ Z + i J scales^ as which has been shown by dotted line in fig.2. Scaling of same parameter with experimentally required laser flux in the actual situation when P^ and P^ are comparable is shown in the same figure with full line. If this scaling is now viewed keeping variation of vs T (in fig.1) ,in mind, it becomes clear that in the temperature range ( and hence the flux range) where radiation losses ? increase with temperature, scaling becomes poorer 4*25 4 /9 ( T ^ ) than ^ as laser energy is not used up completely for heati-ng and is increasirgly lost in radiation. Similarly when losses decrease with temperature, scaling turns out to be higher ( T ~ 2>’^) 4 /9 than rt> • For temoeratures where losses are negligible, scaling T 4 /9 remains justifiably same as (£ . Experiments were conducted by focussing 5J» 5 ns Nd:Giass laser on a carbon target and the ion expansion1 energies were measured using a charge collector. Electron temperatures were obtained using relation between ion expansion energy and electron temperature fo^y^ ■ partially ionized plasma. Experimentally deduced values of as a function of incident laser flux are shown in fig.2 which agrees ■ - 5 -well with the expected behaviour. Though for high Z plasmas , lower scaling of temperature over flux has been obtained earlier, a higher dependence could not be seen as the temperatures reported are such that the P^/P^ does not reach its mai-lmgm in this flux range. However: carbon being a moderate Z target, maxima of P^/P^ falls within this flux range and both the scaling are experimentally observed. Authors gratefully acknowledge the technical assistance rendered by Shri S.E.Kumbhare in this woric. - 219 - References: 1. B.H.Ripin et al, Phys. Fluids,-2£, 1012 (i960) 2. H.Puell, Z. liaturfarsch 25a. 1807 (1970) 3. P.D.Supta, S.K.Goel and D.D.Bhawalkar; "Determination of electron temperature from ion expansion energy in laser produced plasmas'1 (present symposium) 4. G.J.Pert, Plasma Phys. J6, 1019 (1974) 5. B.K.Sinha, Ph.D. Thesis, Bombay University.1980. i 220 - 221 - Eon-saturation Behaviour- of V-I Characteristics Of A Place Langmuir Probe In Streaming Laser-Produced Plasmas 3.K.(Joel, P.D.jupta and.B.D.Bhawalkar Laser Section Bhabha Atomic Besearch Centre Bombay-400085 In stationary plasmas, electron current collected by disc Langnuir probes which are biased positive with respect to plasma potential does not increase^1^ with the increase in probe potential. In case of streaming plasmas produced by lasers, it has been experi mentally observed^-^ that electron current does not saturate but increases linearly with probe potentials even in the cases when debye length is much less than the probe radius. While for cylin drical probes, similar non-saturation behaviour has been explained by Segall and Koopma^) no efforts have been made to explain it for plane disc probes. In this paper, we obtain this nonsaturation behaviour for a plane disc probe using a treatment similar to that of ref.3. Segall and Koopman^ have shown that due to high streaming velocity of ions, plasma fails to shield the positive potential applied on the cylindrical probe and this causes positive potential to extend far into the plasma. Sheath surface which is cylindrical in this case was assumed to extend upto infinity in the flowing plas ma. Similarly for ,a plane flush probe, sheath surface which will be hemispherical at large distance from the probe can also be assumed to extend well inside the flowing plasma. Let a plane disc probe of radius B lie at the centre of such a hemispherical sheath surface of radius a as shown in fig.1. If U is the" flow velocity of plasma moving normal to the surface of the plane probe and l5v , (?z , $ represent the velocity components of a particle in cylindrical coordinates chosen in a manner shown in fig.1 for every point on the sheath surface then at a particular point such that the line joining that point the centre of the plane probe makes an angle & with the normal to the probe surface, Maxwell-Boltzmann distribution will be given by - 222 - v i *,m v = ((,,-uoss)2 + The current arriving at the probe surface from a segment af area 2 TT a2 Sing d @ will be iC©)ole = 2rra2A M i6ddM jrd ^ fdL(9z [ $Luiy f- LKD*#) —fT o a x £ l^C&d - [5YCS54> A i^e] ■ whereis the. largest radial velocity which a particle starting at the sheath edge with an axieil velocity^ , making an angle <£> as shown in fig.1, can have and still hit the probe. For a particular 5 and maximum angle ol which a particle can make with z-axis (fig.1) so that it strikes the probe in a case when probe potential is zero will be given by ( a zt JR2- i-2Q .R ^ © W ) 2 Cl-S*6nGAM'4>) which can be readily derived from geometrical considerations for every point of the sheath surface. For a particular value of (5^ and including the effect of positive potential applied on the probe, L$?y1 , will be given by LSy. - tot/V} d where V is the positive probe potential with respect to the plasma potential. Applying hospital's rule for taking the limit Ct-><0 , we have ~ r f A J% 6)ds aw* 7, = - 223 Pig.2 shows that for this case of plane probe used with streaming plasmas, electron current does show a linear increase with probe potential since with the increase in the probe potential, more particles will be' attracted towards the probe, number of particles collected by this finite dimension probe w ill increase. References;. 1. I.Langmuir ahd K .M .Mot tsm ith, Phys.Rev. 28. 727 (1926) 2. C.T.Chang, M.Eashmi and H.C.Pant, Plasm Phys. _1£, 1129 (1977) 3 . 3 .B .S eg a ll and D.W.Koopman, Phys. F lu id s , 16, 1149 (1973) FIG.1 : OA (O'A1) is the zero angle reference line for angle o ' b , radial component of the velocity w ill make angle 4* ( Z A'O'B') with the reference line O'A1. ( IS*-, ^ 2 * 4* ) d efin e completely the initial magnitude and direction of the velocity of a particle enterirg the sheath from the point 01. i4 U 11 to 9 224 1 5 4 3> 2 2. - 225 - Temperature diagnostics of atreaming plasmas produced by lasers S.K.Goel, P.D.Gupta and D J> .Bhawalkar Laser Section. Bhabha Atomic Research Centre Bombay-400065 As high temperature, high density laser produced plasmas expand through vacuum, their thermal energy gets converted into directed kinetic energy and low temperature, low density streamizy plasmas are formed. Diagnostics of such streaming plasmas is important to know the hydrodynamic expansion and recombination mechanisms1 involved 2 3 during their expansion through vacuum. Cylindrical and,plane Langmuir probes have been used for temperature diagnostics of stream- 3 ing plasmas but temperatures estimated with plane probes are higher 2 than those estimated with cylindrical prohes used to diagnose similar plasmas produced by high power lasers. Due to high flow velocity of the plasma, slope of that part of V-I characteristics which is used for temperature estimation 4 decreases compared to the stationary plasma case. Though this decrease has been theoretically estimated and experimentally incor- 2 3 porated for cylindrical probes , in case of plane probes ,procedure applicable for temperature diagnostics valid only for stationary plasmas has been used even with streaming plasmas resulting in erroneous temperature estimations. In this paper, we rectify the treatment of ref .2 and 3 and present a simple calculation to relate the slope of W I characteris tics 'of plane as well as cylindrical langmuir probes with electron temperature -of flowing plasmas and show that the decrease in the slope of V-I characteristics due to high flow velocity of plasma is more for plane probes than for cylindrical probes. Let y be the flow velocity of plasma flowing in a way shown in fig.1 for disc probe of radius B or a cylindrical probe- of radius R ■ and length 1, U. be the electron velcdty component normal to the probe surfaces in the two cases. Electron currents collected by the probes biased to a negative potential V with respect to plasma will be given by - 226 - fo r plane probej (s y 'w t v ‘bi "i for cylindrical probe; where symbols have their usual meanings. These equations after ppoper normalization reduce to I_- ^ exf>[ -l(Ld-iaef] dLcj TPi ( 1) s r ! ± o U } d a ( 2) l N ) \ T ^ = Ujjg I>IJ& , n - ev/bT . . Pig.2 shows V-I characteristics for two types of.probes. It is to be noted that as probe is biased to a more negative potential with respect to plasma, the slope of the characteristics increases in stead of remaining constant dja in case of stationary plasmas. Slope of the line obtained by a least square f i t in an experimen tally obtained characteristics, hence,■ would depend upon the range of the probe potential upto which characteristic has been taken on the negative side of the plasma potential. Por example if potential range from zero to -5 V with respect to plasma potential is covered in certain experiment, then overestimation in temperature estimated by the slope of the characteristics as done in the stat ionary plasma situation will be as shown in' fig.2 for two types of probes which shows that this will be more in case of plane probes compared to cylindrical probes. It is so because flow velocity remains1, normal to the probe surface in case of a plane probe whitfi its component normal to the probe surface varies between zero and - 227 - U over the surface of a cylindrical probe. References: 1. P.T.Bumsby and J.W.M.Paul, PlasmaPhys. _16, 247 (1974) 2. S.B.Segall and D.W.Koopman, Phys. Fluids 16, 1149 (1973) 3. C.T.Chang, E.Hashmi and H.C.Pant, Plasma Phys. .IjJ, 1129 (1977) 4. I .Langmuir and H.M.Kottsmith Phys.Rev. _28, 727 (1926) Row/NG, Plasma V,UL P i s c P R o a E c y lindricalprobe £ i£H hrc&yjtciye. ovfi Y Q siiv w &H q>i <7/ Oo -1o OM 0 - 2 . X CWavacfevisiics W C -X V 0 - 2 , Ove^esfc‘t0wx.+vo>L or S’ -S C.-8 o-> 1-o ,-2 r o 228 - 229 - Determination of Electron Temperature From Ion Expansion Energies In Laser-Produced Plasmas P.D.Gupta, S.K.Goel and D.D.Bhawalkar Laser Section Bhabha Atomic Research Centre ‘ Bombay-400065 Foil transmission technique^ has been widely used for elec tron temperature estimation in laser produced plasmas. This techni que requires several well calibrated x-ray defectors and judicious choice of foils to make the ratio of transmitted intensities a sensi tive function of temperature, T@ . In comparison, ion expansion energy, Eion> in laser produced plasmas can be measured by a time of flight using a simple charge collector. If E ., can be related to - ion (2) Tg, then this can be used for temperature diagnostics, Puell has derived and used a simple relation E, =5 (Z+1) kT .I... (1) ion • e between ion expansion energy and electron temperature. For moderate laser fluxes (4 lo'^W/cm^) and high Z targets, plasma is not fully' ionized and the relation^1^ needs to be modified. In this paper we obtain the dependence of expansion energy on electron temperature for partially ionized .plasmas and present results of temperature estimation in various targets using this analysis. (2 ) Ion expansion energy is obtained by equating the laser flux with the outward flow of plasma .energy. To take care of the energy spent in ionization, an effective value of 'T?' as defined by (3) Bykovskii is used <• _J 2 Q(Z) , where Q(z) = Z l(Zj) --- (2 ) Y-1 2 (Z+1 )kTe l(Z,) are ionization potentials. As the plasma expands/ the thermal^gets converted to kinetic energy of expansion. Eneigy spent in ionization .reappears as plasma recombines during expansion. This energy is. retained by plasma where 3-body recombination is' dominant process. For this case the relation between E. and T is obtained as ion- e B lon = l n [ZkTe + T J (3) 2? (r-1) 230 - where P is the fraction of laser energy transmitted to the one dimen- ( 2 ) sional surface and is estimated as (4) P” eip h sfc)] For fully ionized plasmas y = J> , = 2 and then relation (3 ) tallies • 3 "3* with Fuel! result i,'eq. (1)) for 2 e=T, i . For one dimensional expansion p =1 and relation (-3) reduces to the one obtained by Bykovskii . When radiative , recombination is important, ionization energy is radiated away and in that case = (z-»1 )kTe £J~ (y)(lfrl) (r)(y»-1) _- t's_ty)T(5-3y)j • - •••• (5) 2(r~i) For high temperature plasmas V-» 5/3 ami /3-y i*/3, this relation reduces to eq.(l). This is so, because ionization e.nergy will be much smaller than the thermal energy firnr Mg h tempesratures 1 and hence can be neg lected. Calculated values of -for carbon pliasma as a function of HT" ■ temperature are shown in fig;.|l). Since these' values for two cases significantly differ in low. temperatuire range, it is important to know the dominant mechanism; rnf recom bination. Relative importance of (4 ) 3 body recombination and radiative recombination, processes is determined by whether SH = ^ ___ is > 1 or < t .... (6) 5 3 x 1 ® ^ <4) ;For £n y 1, 3-body recombination is dominant and remains so througjar out the expansion, and hence relal cion (3 ) is valid. For 1, radia tive recombination remains dominant only upto a distance x _ / -g. (7 ) ;after which 3 body recombination takes oyer. ■ Energy loss due to re combination up to this distances can be estimated and aubstracted from expression (3 ) for obtaining ion expansion energy. Values of Eion thus calculated are also show/n in fig.(l). However for kTe 1, most of ionization energy is lost due to radiative recombi nation and expression (5 ) is valid. - 231 - 232 Experiments were carried out on carbon, aluminium and polythene targets with a 5 J, 5 nsec 'Id:glass laser. . Expansion energies were determined from time of arrival of ions at a charge collector. Temperatures were obtained using similar calculations as depicted in fig. (1). Variation of If 1*2+1 as a function of laser flux, t|), for these targets is shown in fig.(2 ). Scaling'^ of this parameter on (j) as is in accordance with the self regulating model valid in this flux range. Authdrs gratefully acknowledge technical assistance rendered by I.Ir .S .3.Xumbhare in this work. References: (.1) ?.0 .Jahoda, et al., Phy.Rev. jr°, 843, I960 (2) H.Puell, Z. Maturforsch, 2£a, 1807, 1970 (p) Yu.A. Bykovskii et al, Sov. Phy. JBTP 33. 706, 1971 (4 ) P.T.Rumsby, J .W.M .Paul, PI .Phys. _16., 247, 1974* LASER APPLICATIONS Sequential Hologram Interfercmetiy with 233 diffusely illuminated objects - V.G-.Kulkami ana D.Sen. Investigation of strain distribution 236 in a square plate subjected to torsion using speckle-shearing interferons try. - K.V.S.G.Bao and D.V.K.Rao. Effect of laser emission parameters on 240 the transmission capability of optical fibres - V.V.Bampal. Design of multibeam infrared perimeter 244 intruder alann system — B.C.Khattar, S.L.l'akker and U.C.Bhartiya. Periodic surface ripples in laser 247 treated aluminium and their use to determine absorbed power - A.K.Jain, V.N.Eulkarni, D.K.Sood and J.S.Uppal. Remote measurement of density fluctua 251 tions in turbulent flows - J.S.Goela Atmospheric probing by laser techniques 255 - 3.V.Krishna Murthy, P.R.Kadhava Panickerj K.Paraaeswaran, K.A.Radha- ktishnan, M . datyanarayana, D.R.Selvanayagam, 5 .Sukumaran Nair, and K.P.Sundaran. Laser - 'an inertia free tool1, a 257 study with an Industrial Laser System - l.l.Sarkar, S.K.fTikumb and R.T.Shah. Use of Par Infrared Laser for KHD 261 plasma diagnostics - S.V.Deshmukh, S.P.lupta and V.K.Rohatgl. - / ; . '• ’ : -1 • • - 233 - SSQUEITTIATi HOICGRAK IFTERIEECI'.ETRY YITH . DIFFtTSBir IELUI-;IKA$BD C 3J2C IS V. S. KDIiKARiri and D. SEE ...... : Standards Division " national Physical laboratory, Ifev/ Delhi-11Q012l~‘ ■'"The application of the double-exposure and real time holographic- interferonetry to deformation analysis - is 'limited by the fact that if the total range of surface " displacement', .of an'-'obiect is relatively large, ‘the number '■ o f in te rfe re n c e frin g e s in the re c o n stru c te d image becomes " too large for convenient evaluation. In such circumstan- - ces it is advantageous to have a sequence of holograms on •a single photographic plate as the object undergoes pro gressive deformation. The interference pattern between " any two states' of ^the object can be conveniently studied by reconstructing^%wo light fields at a time sequentially. Unfortunately,--, it ^rs not possible to apply the usuial spa tia l multiplexing "technique (1) as reconstructed light f i e l d from one - region o f the re co rd in g medium does, not interfere "with others obtained from different regions \Z y. '* However, by recording two states of the object on cdmmon ' <* <** * * *■ * ~ r ^ . , *♦ „ p o rtio n s o f th e -re c o rd in g medium, th e s p a t i a l multiplexing *" technique has been applied to obtain a sequence of double exposure holograms (3). But the primary draw backs of --•'this multiplexing "technique are the decrease of resolu- tion and the vafrat ion of the direction of observation of '*• the object from one elementary hologram to another which ■ *- can: intro due e errors in the evaluation of the correspond ing disp 1-acements£ In this paper, two simple multiplexing - 234 - ■techniques are described which overcome the above draw backs. Basically, the techniques use a combination. of .theta modulation and carrier frequency multiplexing (4,5) for recording a series of image holograms. The technique of multiplexing with single reference beam uses a small angle prism to vary the direction of the reference beam between successive exposures. A dou ble exposure hologram corresponding to the two states of the object is recorded at each position of the prism. The, holographic plate is developed in place and'the interfe rence pattern corresponding to the displacement of the object surface between the two states is obtained by ill* minating the hologram with the corresponding positions ,of the prism sequentially. The reconstructed wavefronts ,from the hologram propagate along the axis of the object beam for all positions of the prism and therefore the direction of observation does not vary froin one elemen- 'tary hologram to another. The technique is successfully .applied to obtain a series of interferograins of a diffu sely illuminated transparent object under 'different loads! i The interferogram series shows the total phase changes' for each of the loads as well as .differential phase chan ges between the two loads and consequently large phase differences can easily be analysed. The technique of multiplexing with multiple refe rence beam uses a separate reference beam for recording .each state of the object independently. An exposure is - 235 - made of the object in its initial state with beam as a reference. The object is then stressed and another expo sure is made w ith beam R^ as th e re fe re n c e . S im ilarly , the successive exposures are taken by increasing the stress and changing the reference beam. After processing, the hologram is illuminated by the original reference beams two at a time so that the interference pattern cor responding to the displacement of the object surface bet ween any two states is obtained. This multiplexing tech nique can also be used for dynamic studies by accomplish ing the fast switching of the reference beam between the exposures. These techniques are quite useful in deformation analysis and especially in the investigation of the dimen-j- sional evolution of the object displacement, variations etc.. ^REFERENCES 1. H .J. C a u lfield , App. Opt. 2., 1218 (1970). 2. R. Dandliker, E. Marom and F.M. Kottier, Opt. Commun. 6, 368 (1972). 3. . P. Hariharan and Z.S. Hegedus, Opt. Commtin. £, 152 ' ( 1973,). 4. V. G. Kulkarni, P.N. Puntambekar,- D. Sen and K. De, Opt. Commun. 27, 214 (1978). 5. V. G. Kulkarni and P.If. Puntambekar "Holographic Multiplexing using multiple reference beams" (To be p u b lish ed ). - 236 - : INVESTIGATION OF THE STRAIN DISTRIBUTION IN - A SQUARE PLATE SUBJECTED TO TORSION . USING SPECKLE-SHEARING INTBRFEROMETRY Gnaneswara Rao. K.V.S. and Krishna Rao. D.V. Physics Department, Andhra U niversity Visakhapatnam-530 003. INTRODUCTION The present paper describes application of a recent speckle-shearing interferometric method reported by Hung and Liang'1" for the determination of strain distribution in a square plate subjected to torsion. This problem is of practical importance in the design of structures and machine parts. The speckle phenomenon has been u tilis e d by sev e ral investigators for the measurement of surface displacements and for the direct determination of derivatives of surface displacements THEORY OF A SQUARE PLATE UNDER TORSION The shearing strain along the y-axis in this case follows from the existing analytical solution^ as : ^zx = 't'zx (1) where Tzx = corresponding shearing stress s- r Poisson's ratio of the material of the specimen and E - Modulus of elasticity. Similarly the shearing strain along the x-axis is: ^zy s * %y ^ IMAGE SHEARING CAMERA With normal illumination ( 0=0), the slope of the scattering surface in the direction of shear is given by: dw (2N+1) X “3x“ r TP 3 where : Sx = image shear in the x-direction X = wavelength of the light used Dark fringes occur when N = 0,1,2,3 .... The experimental value of shearing strain can be obtained as : £ zx = ~T " ii " ...... W similar expressions follow for the slope dvr/ by and the shearing Strain 6gy in the y -direction. Fringes can be re-imaged using a white light Fourier- filtering technique described by Jones and Leendertz^. The resulting fringe system yields the slopes of the displacement directly. EXPERIMENTAL DETAILS The object was illuminated by a point-source of coherent light from a He-Ne laser as illustrated in Fig.1. A twisting moment of 286.16 kg.cm was applied to a square Perspex plate (length = 5-08 cm) under an image shear of 1 .9 4 2 mm. Plate 1. shows slope contours along the x-dire- ction in the form of rectangular hyperbolas. Plate.2. shows equally-spaced parallel fringes along the y-direct- tio n . The p atte rn s were analysed and the re s u lts compa red -with.the theoretical_yalues as shown in Figs.2 and J. - 238 - DISCUSSION AND CONCLUSION Reduction of spurious speckle noise and enhancement in the clarity of the fringes was acheived by white light filtering. Fairly good agreement between the experimen tal and theoretical values shows the'reliability and accuracy of the technique employed. CAPTIONS TO FIGURES AND PLATES Fig.1. Schematic diagram of the optical set up. Fig.2. £zx . vs. y along the y-axis of a square plate under torsion. Fig.3'. £zv .. vs. x along the x-axis of a square plate under torsion. Plate.'1. Fringe pattern depicting ^w/ b x on a square plate which has suffered slight warping1. Plate.2. Fringe pattern depicting dw/ by on a square plate which has suffered slight warping. REFERENCES 1. Y.Y.Hung, C.Y.Liang, Applied optics, 18(7) . 1046-1051 (1979). 2. R.K.Erf. Ed., Speckle Metrology (Academic Press, New York, 1978). 3,4. K.V.S.Gnaneswara Rao, D.V.Krishna Rao (To be publi shed in 'A tti Della Fondazione Giorgio Ronchi1 Italy). 5. Chi-Teh. Wang. Applied Elasticity, (Me Graw—Hill Book Company, New York, 1953)* 6. R.Jones, J.A.Leendertz, J.Phys. E. (Sci.Instrum) Z, 616 (1974). * * * - 239 - Coherent Object z Glass plate Image shearing Camera FIG. 1 F I G . 2. FIG. 3 For plates 1 and 2 please see plates E and F at the end of this book. -240- Effect of Laser Bnission Parameters on the Transmission Capability of Optical Fibers. Er.V.V.Rampal Defence Electronics Applications Laboratory Dehra Dun 248001 Loss and dispersion are the two main considerations for the design of a high data rate fiber communication system. The total 1 power available to the system is given by F (T) = K, . - Kg U .T '1 + P ...(1 ) there T is the bit rate interval and P is the launched power and K^, Kg are constants. This available power F(T) must compensate for all the losses in the system which include intrinsic loss per Kin and the power margin for error free operation. Launched power, bit rate and length are therefore interrelated for given value of loss and dispersion for the particular cable. During the last decade considerable progress has been made in reducing the transmission loss, and, minimum loss can be expected to be 0.2 dB/Km^”^. Dispersion can be considerably reduced by adopting the single mode operation so that contribution due to modal disoersion is almost eliminated. The dispersion in single mode fiber is chiefly composed of material dispersion and waveguide effects and depends, among other things, on the wavelength'X and bandwidth 5x of 1he source radiation! It can be shown that for a length L of the fiber, the single mode 5 dispersion is given by ., v ' T >- 1 ^ ^ ^ 2 T rlj/b - 1 ______12) where n^,ng are the ref. indices of Core and Cladding, C is the velocity of light and A , b, v have their usual meaning. Fig-1 shows a plot of eqn (2) for a given fiber with Getig doped -241- core and pure silica as cladding and core dia.~et.er = 5 ps, A - - \ It is seen that both for graded index (c< = 2) and step index, fibers the total dispersion D — *- 0, The value of z\c ( Xat D = c) is significant as operation at this '.wavelength leads to very high data rates. In practical situation hov;ever it is not very convenient to design a fiber so that its 3i6 matches with the source wavelength or to choose a source so that its wavelength exactly coincides with )*0 for the fiber, that is practically convenient is to select a source and fiber such that the operation is near T'o . One can then estimate the effect of dispersion due to £>/ and shift of /\ and see whether this dispersion is within the allowable limit imposed by date, rate consideration. The following example will illustrate the point. The response time of the fiber system is given by t =1.1 ( tg + ^ + t^ )^ ■ ... (3) where .s,f,d refer to source, fiber and detector respectively. For digital transmission, t. = K — where K = 0.7 for KHZ format ...( 4) data rate „ „ . = 0.35 for Rr. format For data rate =1.5 GHz in KRZ format eqn (4 ) gives t = 0.5 ns. If source and detector each have response time of 0.3 ns, eqn (3 ) gives t^ <- IpO ps. .From fig-1, for X = Xo +. 0.1 pn the total dispersion D is 7ps/nmKm for the step index and 5 ps/nmKm for the graded index fiber, If 5S = 5nm, this would give 50 - 70 ps dispersion for a 10 Km repeater spacing which is well within the limit of 1 5 0 ps obtained above. =242= 1 - JE Midwinter, Optical fibers for transmission, M l e y .1979, P570 : . 2 - T. Izave, N.Shi’oata, A. Takeda, Appl. Physics Letts,31,53, ; : 1977 - ' . 5 - T. Miya, Y.Terununia, T/ Hosaka, T. Miyashita,Electron. Letts,15,106-108,1979 4 - . VM. Gambling, H. Matsumura, CH Ragdale, " IEE J. Microvave Opt. and Acoust 5,239 - 2 4 6 , 1979 5 - V.V. Raiapal,Zero dispersion optical fibers for high data rate system, Def Sci. J ( to be published ) -243= 55 a VS - r - -3o- D ISPE R SIO N VS WAVELENGTH OP A SIN6LE MObC C SI i-1C A BASEbJ) Fib r e Za-.sp™, & - '% -2 4 4 “ Eesi^n of Multibeam Infrared Perimeter Intruder Alarm System R.C.Khattar, S.L.Makker, (J.C.Bhsrtiya Laser Section Bhabba Atomic Research Centre Trombay, Bombay- 400085 The need of perimeter type intruder alarm system arises in security problems of installations like ammuni tion depots, prisons, Storeyards, airports etc. Such systems should not give spurious alarms as they cover a ■ v/ide area. They should be tamper-proof and should not give alarm when any intruder tries to defeat with any other similar source. There are different types of alarm systems available working on different principles. We have designed an infrared multibeam intruder alarm 'system. The units of the system are small and inexpensive. ,The transm itter uses laser diode as infrared radiation ■source. A single beam can leave sufficent Vertical space for the escape of an intruder. Three or more beams are :used to make one system and 3 or more such systems are required to provide security to any area. The s*et of transm itters and receivers are mounted on two posts facing each other and in the line of sight. Laser diode is used in each transm itter as it is an excellent source of infrared high intensity radiation in small size with low ■divergence. A specially developed circuit for this system is the high current short duration current pulser which drives current pulses to semiconductor laser diode. This pulser uses locally available transistors which are used in avalanche mode to give pulses upto 18Amp for about 200 cSec.duration. The laser diode gives an output of about 4 waits at this current. Bach transm itter covers 3 receivers with the radiated 1.R beam. These transm itters are pulsed in sequence to cover the complete height of the security wall. A unique method has been used to defeat any -245- attempt by the intruder to capture the receivers by external 1.R source. In the event of an attempt to cap ture, alarm is raised. The receiver uses photodiode in the photo-vol1&c mode. The detected signal is amplified by first three stages of IM 324 type integrated circuit. The 4th stage has been utilized to,amplify the AGO signal. The AGO circuit takes care of the loss in signal due to fog, .smoke or rain to the extent of 13dB. The output of each receiver amplifier is connected to the time- sensitive alarm control logic circuit which gives an alarm with the following conditions. ( i ) 90 % of any two out of 3 b§ams are blocked by an intruder for more than 75 m.Sec. Or (ii) 90# of any one of the 3 beams blocked by intruder for more than 1.25 seconds. The transm itters and receivers are fixed at the heights of 20cms., 90cms, and 150cms above ground. A concrete base is prepared along the perimeter to avoid tunneling through, A deep red filter in series with the receiving optics is used for attenuating visible light during day time. Rotating mirrors have been provided in each transm itter and receiver for • ease of optical alignments in field even if posts are not exactly vertical. The ,beam is wide enough not to cause any false alarm due to vibrations produced by heavy vehicles movements in the v i c i n i t y . 246- BLOCK DIAGRAM OF A THREE TIER INFRARED INTRUDER Al ARM SYStFM “ TRANSMITTERS x 3) RECEIVER ( Rx I ) il-R-SOURCE — transmitting Receivin g photodiode LASER OPTICS OPTICS AMP L. i 1 DIODE A.G.C. air path u p to 150M T x-2 I R x-2 GENERATOR I. R. SOURCE TRANSMITTING RECEIVING PHOTODIODE TIMING AND L^SER AMPL.& THRESHOLD OPTICS OPTICS CONTROL TRIGGER CKT. DIODE A.G.C. CKT. T x-1 Rx-3 PHOTODIODE VISUAL VOLTAGE I.R.SOURCE TRANSMITTING RECEIVING CONTROLLING LASER -* -*• AMPL.Se AND OPTICS CKT. .DIODE a . g:c . SOUND ALARM POST 1 POST 2 1 T.RAHSMITTER) ( RECEIVER) BLOCK DIAGRAM SHOWING BEAM PATTERN FOR THREE TIER INFRARED INTRUDER ALARM ..SYSTEM . -247- p e r i01 iG,- Surface r ip p l e s in la ser treated ALUMINIUM AND, 'THEIR USE ,TO .DETERMINE ABSORBED POWER ’ Animesb K .' Jain7.,'v«N. Kulkarni*, D.E'i* Sood and J.S. Uppal** Nuclear "Physi'cs; Division, Bhabha Atomic Research Centre, . Bombay - 400085 (INDIA) Periodic’surface" Pinnies with an average snacing * . . '""'I1- :» matching'with'the incident’wavelength, A have been reported for CW ’^>3 as well as p u lsed * laser treatment of semicon ductors . These fringes are believed to arise from a stan ding wave on the"siurface produced by interference of the incident laser beam with a ..coherently scattered longitudinal wave from a surface disturbance. The standing wave gives rise to intensity variations at the surface which produce periodic melting when the power density is near melt thre shold . Thus the appearance of such "characteristic ripples may be treated as the 1 signature1 of melt threshold. We report-the first observation of such ripples produced on'a metal (pure and ion implanted Al) surface. Furthermore-we propose a method to determine the absorbed power by comparing the experimental melt threshold (delinea- -y g V ted by ripple zone) with the calculated melt threshold ’ . This•technique provides 1 in situ* evaluation of absorbed powter on each laser treated specimen,, and would surmount difficulties associated with variations in reflectance caused by surface preparation procedure, surface oxides and ion implantation etc. Pure as well .as ion implanted polycrystalline alumi nium samples were chosen for the present study. Details of ' iinolantetions: are.-given in table I. Laser treatment was done • ~ X x • \ ' W :...... - - t- * Research Fel^o’w, Marathwada.University Aurangabad. ** Laser Section^ -248" with a Nd: Glass laser (X = 1=06 pm) operating in a TEM mode with pulse duration of 7 ns ?WHM. Bach specimen held- vertically was irradiated in air with a single pulse at normal incidence. Surface topography was studied with an ETEC Scanning Electron Microscope in secondary electron mode. T/e observe that fringes are present at/just outside the borderline of the laser spots. Fig.1a shows the fringes on a pure A1 specimen - #he laser spot (5-5 mm 1/e dia, 8.4 j/cm2 peak) periphery is located about 20 pm to the left of this region. Fig.1b shows the fringes observed on an A1 17 specimen implanted with 400 keV Cr ions to dose of 2x10 ions/cm2 - the laser spot (2.5 J/cm2 peak) periphery is about 100 pm away from the centre of this region* The ave rage spacing between these fringes is 1 pm matching well with the laser wavelength. These ripples appear only within a narrow region (about 100-150 pm wide) around the periphery. All other implanted A1 samples show sim ilar ripple patterns. If the measured radius at the melt threshold (the centre of region showing ripple staj^cture), the energy density at observed melt threshold, - £e CXp(“ /'VJ'1) is determined frofe known values of the peak energy density £ e and l/e diameter of the Gaussian laser spot. The effe ctive reflectance is then n welt ^ = 1 ~ (^calc ebs / where is the calculated absorbed energy density at which the pure material just melts. can be readily cal culated from standard one dimensional heat flow treatment®’"^ and is 0.16 j/cm2 for A1 for our laser pulse. We have used this method to determine for pure and ‘implanted A1 and the results are shown in Table I. The Bgfffor pure A1 is in excellent agreement with the reported® reflectance of 0.94. Ion implantation produces marked variations in ®eff .f°r A1 and maximum change is seen for Ou ions implanted at' 400 keV 17 2 to dose of 5.7x10 /cm . Table I shows that the type of im planted species has more pronounced effect on than that of dose variation. T ab le I Pulsed laser treatment of Aluminium. Overall error in i s 1*. Implan- Dose 2 E0 - 2 w 2 m e lt Ref'f ted (Ions/cm )(J/cm ) \mm) (mm) ®obs S p e c ie s Hone - 3 .2 3 .6 1.56 2 .7 0 .9 4 Sb, 75keV 1 .5 x 1 0 16 3 .7 3 .6 1.21 3 .3 0 .9 5 75keV 6x1016 3 .6 3 .6 2 .0 0 2 .7 0 .9 4 30k eV 1 .3 x 1 0 17 3 .2 3 .6 1.86 2 .5 0 .9 4 Cr,400keV 7x1016 2 .2 3 .6 1 .42 1 .9 0 .9 2 400keV 2x1017 2 .3 3 .6 1 .52 1 .9 0 .9 2 Cu,400keV 1 .7 x 1 0 17 2 .3 3 .6 1.47 2 .0 0 .9 2 400keV 3 .7 x 1 0 17 2 .3 3 .6 2 .03 1.7 0 .9 0 For Fig 1. Please see plates G and ti at the end of this book. = 250- R e fe re n c e s 1. D.C. Emmony, R.P. Howson and L. J. W illis, Appl. Phys. Lett. 23, 598 (1973). 2. J.S. Williams, W.L. Brown, H.J. Leamy, J.M. Poate, J.W. Rodgers, D. Rousseau, G.A. Rozgonyi, • J.A. Shelnutt and T.T. Sheng, Appl. Phys. L ett. 21« 542 (1978). 3. G.A. Rozgonyi, H.J.'Leamy, T.T. Sheng and G.K. Seller, in "Laser-Solid Interactions and Laser Processing - 1978, S.D. Ferris, H.J. Leamy and J.M. Poate (eds.), AIP Conference Proceedings No.50 (1979) p . 4 57. 4. H.J. Leamy, G.A. Rozgonyi and T.T. Sheng, Appl. Phys. Lett. 32, 535 (1978): 5. M. Oron and G. Sorensen, Appl. Phys. Lett. 3£, 782 (1 9 7 9 ). 6. P. Baeri, S.TJ. Campisano, G. Foti and E. Rimini, J. Appl. Phys. 50, 788 (1979). 7 . Animesh K. Jain, V.N. Kulkarai and D.K. Sood, Appl. Phys. (to be published). 8. Alexander Goldsmith, I.E. Waterman and H.J. Hirschhorn (eds.), Handbook of Thermophysical Properties of Solid M aterials. Vol. I s Elements (Macmillan Co., New York, 1961), p.43. -251- RM 0T2 MEASUF.S1EUT OF D 2ISH Y FLUCTUATIONS IN TUKBULEIT FLOWS Dr. J.S. Goela Department of Mechanical Engineering Indian Institute of Technology Kanpur Kanpur 208 016 A new three-level gain measurement scheme is theore tic ally examined with the aim of providing important information about the density fluctuations in a turbulent flow field non-intensively, instantaneously, accurately and. remotely. The basic three-level gain measurement scheme entails using a high powgr pulsed laser to satu- • rate one transition of a molecule and measuring gain on a coupled transition of the same molecule with a probe' laser. As shown in Fig. 1 a, the pump laser saturates the transition 1 *-»2 while the probe laser measures gain on the transition 2 *-*3. If the pump laser intensity is always greater than the saturation intensity for the transition, pump laser variations play no role and satu ration of the transition is always assured. Saturating -transition 1*-*2 means that a specified number of molecules (and this number can be calculated readily) w ill be excited from level 1 to level 2. Mea surement of small signal gain on transition 2«-*3 can provide quantitative information about the number of particles in level 2. This in turn is related to the total concentration of the species through the saturation -2 5 2 - condition. To obtain good spatial resolution required for making measurements in a turbulent environment, the pump and probe lasers are placed in a perpendicular configuration as illustrated in Fig. 1b. Since the diameter of the pump and probe laser beams can be made fairly small, the spatial resolution achievable from this technique is only lim ited by the gain of the medium. Information about the density fluctuations can be obtained by splitting the probe laser beam into two parts by using a beam splitter as shown in Fig. 1b. These two parts which may not be of equal intensity serve as two probe beams and measure the gain of the saturated gas at two different points in space. By varying the dis tance between these two beams, the spatial autocorrela tion coefficient can be determined. If the turbulence flow field is random and isotropic, by taking the Fourier transforms of this autocorrelation coefficient, the energy and the power spectrum of the flow field can be obtained. In the case of a flow field in which the turbulence is not completely random, insteal of frequency spectrum information, a probability distribution func tion can be generated. The species which may be used for three level g a i n measurement include C02, KgO and HF. The proposed density fluctuation measurement scheme may be used to -2 5 3 - monitor turbulent density fluctuations behind an airplane, in the diagnostic studies of MHD flow channels and in the measurement of spatially resolved gain in high power HF-lagers. =>254“ 23 1 a The basic three level gain measurement scheme Defector 1 Defector 2 PUMP Laser Z V , Mirror Beam Splitter Collimolion Optics Probe Laser 1b Arrangement of pump and. probe lasers In the three level scheme to make spatially resolved measurements. -255- ATMOSPHERIC PROBING BY LASER TECHNIQUES B.V. Krishna Murthy, P.R. Madhava Panicker, K. Parameswaran, N.M. Radhakrishnan, M. Satyanarayana, D.R. Selvanayagam, S. Sukumaran Nair and K.P. Sundaran. Space Physics Division, Vikram Sarabhai Space Centre, Trivandrum 695 022. + ♦ + Laser technique of atmospheric probing provides a unique method of observing the atmospheric structure and its temporal variations on a continuous basis. Continuous ob servations on atmospheric density, temperature and turbu lence parameters from a single location are not possible with other methods such as rocket or satellite borne pay loads. In the ground based laser technique, laser is used as transmitter either in a pulsed into or a C.W. mode. The laser energy is directed into the atmosphere by a suitable optical system. The atmospheric molecules and aerosols cause scattering and absorption of the laser energy.. A small fraction of the energy scattered by the atmosphere is collected by a suitable optical system and is detected by a photo-device. The detected signal strength contains inform ation on the presence, range and concentration of the atmo spheric particles. At Space Physics Division, VSSC, two types of laser techniques have been developed. One makes use of a high power pulsed ruby laser and the other, a Argon Ion laser operation in C.W. mode. Using these two techniques, experi ments have been conducted to study atmospheric aerosols and the atmospheric density by detecting the Rayleigh and the scatter components. The ruby laser system consists of a ruby laser as the transmitter with a pulse energy of about 5J. The re- = 256= ceived Signal is detected fay a photomultiplier with suit able amplification. The transmitting and receiving optics are so oriented as to receive backscattered signal. . The C.W. laser system is operated in a bistatic mode to study the variation of the received signal strength with scatter in g a n g le . In the experiments conducted with Argon Ion laser, the laser beam was directed vertically up. and the elevation angle of the receiving telescope was varied from 20° to 85° in steps of 5°. The horizontal separation between the transm itter and receiver was. about 430 metres. : This cor responds to an altitude of about 5 Km. for the scattering volume corresponding to the elevation, angle of 85° of the receiving telescope. Preliminary results of the experiments conducted along.with the details of the two laser systems developed will be presented. EXTENDED ABSTRACT NOT RECIEVED i LASER - '* AN INERT IA FREE TOOL" , A Study with An Industrial Laser System G.G.3ARKAR,- S.K.KIKUHB & R.T.SHAH Research & Development Centre, Jyoti L td., Vadodara - 390003. In the present age of technological competence, large segments of industry have acknowledged the laser an effective and cost saving tool to be used for' machining of materials. Mot only does the laser prove to be more efficient, it can also allow new design concepts to be utilized which were unaccept able due to conventional processing techniques. The industrial laser, capable of delivering enormous thermal energy with pinpoint precision, can be found performing a host of m aterials treatment applications including cutting, drilling, engraving, scribing, m aterial removal, welding and surface hardening. In the ongoing paper, several specific applications, which demonstrates the capabilities of 50 Watt (CW/Pulsed) COg laser in a variety of m aterials such as plastics, ceramics, glass etc. are discussed with special reference to the commercially available Jyoti*s Industrial Laser System, model JLS-C-101. -258- The Laser The Industrial Laser System, model JL3-C-101, designed and developed at the Jyoti 3 & D centre, consists of the laser head, power supply, control unit, gas handling unit,, work table, a versatile beam delivery set and a tar getting unit which forms the complete integrated system. The laser operates in CW mode with 50 watts of conti nuous power and about 250 watts peak power in the pulsed mode. The control panel includes pulse-width variation from IOOjjus to 1 sec. and frequency variation from 1Hz to 1000Hz. Interlocks are provided for safety operation. Applications:- A variety of applications using this laser such as Resistor Trimming, Plastic Machining, Ceramics D rilling and Scribing, Glass cutting have been carried out. ‘ Resistor Trimming:- Trimming thin carbon and metal films on ceramics cores.(helical trimming) by vaporizing lines partway into the resistor area and monitoring in real time the changing resistor value as the trimming is performed. By providing the feed back control from the measurement system to the laser, the laser's output power was controlled. This was accomplished through a digital circuit in -259- the control panel. Plastic Machining:- D rilling ana. cutting operations on acrylics and plastics were carried out. The kerfwidths of the order of 0.5mm were achieved with cutting rates of 50 cm/min and 1 5 cm/min were reached for 5mm and 10mm thick acrylic sheets respectively. The cut surface was smooth and fire-polished so that no. after cut dressing was required. Burr free drills with minimum diameters of 0.4mm were obtained in 10mm thick acrylics.t The heat affected zone was also found minimum. Ceramics D rillingT he ability to drill holes through ceramics is 35 finding use in electroni 30 T =4-o wim cs industry. A variety df types of ceramics were tow “ 2«l- tried for drilling c j *l operation using pulsed & ft 10 COg laser and the results are Presented. It is observed that with %f3. i 4 » quality of ihe holes can be controlled. As the pulse width increases the drill-tim e also increases. A graph Pig.I of pulse-wiath and d rill time shows the linear variation. Machining of Glass:- Cutting operation on pyrex and tuhelight glass (with thicknesses less than 1mm) was performed. Glass ampules were also cut with the pulsed CO^ laser. 'The superior quality of cuts eliminates additional finishing operation. Scribing of IC wafers: Laser scribing of alumina wafers is another application with a. much wider scope. Alumina wafers and ceramic substrates were scribed with CC^ laser. Smoother and narrow kerfwidths were obtained with rates much faster than conventional methods. Acknowledgement s:- The authors acknowledge with pleasure the co-op eration of their all colleagues in general and Mr.7. Haghavan, ir.A .S. Arekar, Hr.D. Chavda and Mr.S.S. Mistry in particular. Authors are also grateful to the management of Jyoti Limited, Vadodara for allowing them to present this work. • -261- Use of Far Infrared Laser for WO Plasma Diagnostics S,l/, Deshmukh,- G.P, Gupta and V.K, Rohatgi WO Project,- BARC,- Bombay 400085 Alkali seeded combustion product plasma is used as conductor for Magnetohydrodynamic (MHO) energy conversion-^-). In the core of WO channel*- electron temperature (Te) is about 2B00-3000°K and ele ctron density (ne) about lO^-1* cm—3 to obtain required plasma conducti vity, Conductivity ( 0 ) of plasma is' to be known very accurately since it is required to estimate electrical power output and the electrode voltage drops. Conductive and inductive probes are used for direct measurement of conductivity. But these methods are subjected to large errors due to high temperature and velocity of gas flotu in uihich probe is to be put physically2^. Alternatively,- 0 can be determined from the relation 0 = ne e 2 /me v,- where v is the effective electron coll ision frequency, MHO plasma conditions also make it difficult to use Langmuir probe for direct measurement of ne, Nonintrusive diagnostic techniques are,- therefore,- required for such measurements. Phase shift measurement by interferometry end Faraday rotation measurement are the techniques for measuring ne with higher accuracy^) Both these techniques require highly monochromatic radiation source like a laser with angular frequency o> > Wp*- where < 0p is plasma frequ ency,- so that the radiation propagates through plasma. It is necessa ry that to 3> mp so as to obtain measurable phase shift. This rules out the use of visible and infrared laser for MHO plasma with Op ^ 5 x 1 CA1 s e c - l . -j-0 have phase shift within the lim it of its linear proportionality with nE,- it is required that o > 2t»p. To avoid heavy attenuation of radiation by plasma,- it is required that o 1^10 Op,- i,e, o -^yiol 2 sec-1 for mhq plasma diagnostics. Putting all these conditions together,- a laser source in far infrared (FIR) region is required for this purpose. Incidentally*- for using Faraday rotation technique with WO plasma*- the wavelength X,- of the linearly plane polarized source should also be in FIR region. In addition,- X should be such that the right and left handed circularly polarized components of radiation propagate through plasma. Effective collision frequency v can be estimated by measuring collisional attenuation of FIR radiation by plasma. Availability of lasers in FIR region have made it possible to use above mentioned techniques for MHO plasma. O ptically pumped FIR lasers*- which are now commercially available,- are particularly more suitable for this purpose. Their significant advantage is that a number of descrete wavelengths can be obtained from the same system by putting different chemicals or even the same chemical'* in FIR resona -tor and by changing the wavelength of pump (generally CO 2 ) l a s e r . Such system, gives polarized FIR output which is nftich more stable than gas discharge type lasers^ Interferometric technique can be used even if magnetic field is -262- not applied, For using this technique when magnetic field is applied far energy conversion,- the .probing beam has to be propagated perpen dicular to the field and ordinary component of the beam is to be used for measurements. Accounting for collisions,- the phase shift is g iv e n by3) = ape2,ne, L fMks um-ts) , Z1H.0£»C ( g 3l4D 2) where ne is the average value of ne over plasma-length L, Michelson or Mach-Zehnder types of interferom eters are generally used for measu ring phase sh ifts, UJe consider use of an optically pumped Methyl F lo u rid e (CH 3 F) laser which gives a strong line at 496 pm. For this radiation and for the double pass Michelson type, interferometer,- with ne = lO^1* cm™3 and L = 5,0 cm*- the fringe sh ift 0 / 2 1 1 is equal to 2,2 which can be measured very accurately. Presence of magnetic field for MHD conversion makes it possible to use Faraday technique. The. probing beam has to be propagated para llel to magnetic field by passing it through insulating walls of channel. Under the conditions of MHD plasma,- Faraday rotation is g iv e n by • 8 . = 2,62 x 10**^ X 2 ne L.B radians. For ne = '10^ cm“3y X = 496 pmy L = 5 cm and B = 2 Tesla B ■%> 3 6 ,5 ° , Collisions! attenuation of the beam by plasma is given by3^ Measuring £ and a,- v can be determined easily. For 496 pm radiation in MHD plasma with v Ci^lOll sec"4,- a would be of the order of 3,7 x 10”^, Thus,- using the same laser ne and v can be estimated. Using these experimental values of ne and v,- a can be calculated within the required accuracy. The .MHD plasma is bounded by boundary layers on the electrode and insulating walls in which electron density is much smaller than in core. The density profiles are approximately trapezoidal. The prob ing beam has to pass through regions of varying density or refractive index, tie have studied the effects of this index variation on. propa gation of the beam through MHD plasma, hlhen the beam is incident normal to the different, layers of varying refractive index*- the refra ctive effect w ill be absent. It" Is- also seen that in actual "experi ment the beam w ill be passed through the central region of the channel uhich u ill avoid the deflection of the beam because of gradients . normal to propagation, , In conclusion,- the beam propagation u ill not be affected by electron density variations in MHD channel. R e fe re n c e s 1) L^H, Rietjensf Phys, in Techno 10.- 216,- 1979, 2) I0M, Gaponov j- L,P» Poberezhsky and Yu. Go Chernov,- Combustion and Flames 23.- 29,- 1974, 3 ) mJ r, Heald and C0B, Ulharton,- Plasma Diagnostics ulth Microuavesf Wiley (1965) Ch,l. 4) T,Y, Change IEEE Trans on Microwave Theory and Tech. MTT-22(12) f 9 8 3 ,- 1 974. PLATES A and B. Pig 2 (a) of paper no,22 Group: Lasers A. W ith o u t pumping B. With pumping ' C. Fig,5 of paper no.23 Group: Lasers D. Fig.2 of paper no.4 Group Laser Spectroscopy.Oscillo gram of the decay curve of CaS: Cu (0.1%) E. Plate no.1. of paper no.2 Group: laser Applications F. Plate No.2. of paper No.2 Group: Laser Applications G. and H. Fig.1. of paper, no.5 Group: Laser Applications SEM micrographs showing periodic ripple pattern observed on a) Pure Al, b) A1 impanted with 400 Kev Or ions to dose of 2X10^/cm^. -H-++- —t-f-t' M-H 1 H-H- 10 |im, AUTHORS INDEX Abbi, S.C. 119, 167 Aggarwal, 3 .S. 1C2, 131 Anantha Lakshni, P. 102 3 Babu Ran .134 Bhalekar, B.S. 30 Bhar, 3.C. 146 Bhartiya, U.C. 244 Bhatnagar, 3.S. 109, 115 Bhatnagar, R. 169 Bhattachaiya, K. 160 Bhatti, H.S. 176 Bhawalkar, D.D. 66,217,221,225,229 Bhide, V.G. 197 Biswas, D.J 41 Borgaonkar, H.P. 123, 163, 166 C Chakrapani, 3. 26, 78 Chandra, K . 99 , 112 Chari, R. 78 Chatterjee, U.K. 33, 37, 41 Chitnis, V.T. 19 Chowdhury, M. 160 D Dahiya, H.S. 19 Dandawate, V .D. 22 Dasgupta, K. 74 s D'cunha, H. 180 Deahmukh, 5.V. 261 Deshpande, A.V. 48 Dev, S.B. 208 Dhareshwar, L.J. 82, 86 D 1 souza, M. 106 D'aouza, B. 106, 116 ^ G Ghoah,. G.C. 146 Ghoah, P.S. 146 Goel, S.K. 217,221,225,229 Goela, J.S. 251 Goyal, M.L. 126, 142 Gopalawamy, N. 213, 215 . Gupta, A.K. 160 Gupta, B.l. 45, 82 Gupta,' G.P. 261 Gupta, P.D. 217, 221, 225, 229 1 Harl, V. 26 Hirlimann, C, 167 1 . " Inamdar, A.S. 123, 183, 186 Itagi, S.V. 66, 70 Itagi, .V.V. 1, 30, 56 J a in , A.K. 247 J a ta r, S. 197 Joshi, A. 180 Job, V.A. 151, 180 K Kart ha, V.B. 180 Karve, H.S. 157 Khan, A*H* 1 Khattar, B.C. 244 Kothari, B.C. 119 Krlahnamurthy, B.V. 255 KU'ahan,S. 213 Kukreja, L.M. 45 Kulkami, A. 66, 70 Kulkami, V.G. 233 Kulkami, V.N. 247 Kumar, A. 106 ’ L Lai, C. 8 , 11 Lavande, S.V. 91, 95 M Makker, S.L. 244 Masilamani, 7. 59 Meenakshi, S. 135, 139 Mehendale, S.C. 137 Mehendale, N.Y. 123, 183, 186 Mlstry, J.B. , 116 Mittal, J.P.. 154, 157 N Kair, L.d. 74 Nair, S.Sukumaran. 255 t Kair, Bpnikriahnan, H .T. 176 Narayan, B.S. 45, 86 K ath, A .K. 33, 37 Hatban, T.P.5. 82, 86 * Nikumb, S.K. 257 Nundy, U. 33 0 Ohtauka, Yoshihiro. 204 P Padhye, M.R. 48 Pandey, A. 154 Panicker, P.R.Madhava. 255 Parmeswar=»j,K .P. 255 Partharasarathy, V. 154 Patel, N.D. 180 Pawar, 5.H. 56; : Prakash, R. 99 , 112 Prakash, Hari. 109, 115, 134 Puntarabekar, P.N. 19, 197 Puri, R.R. 91, 95 H Radhakrishnan, N.M. . 255 Raju, Bh.A.R.B. 78 Rampal, V.V. 240 Sac, D .S. 174 Rao, D.V -Krishna 2*6 Rao, X.V.S.3* 2 )6 Rao, K.V.S.Raisa. 15^.^ 157 Rao, C.Ramachar.dra. 174 Rao, P.Raaaaohana.. 26 Reddy, B.S. 170 Hohatgi, V.X. 261 Boy, D.S. 160 Rustagi, K.C. 1)5, 1)7, 1)9 S Santoso, Budi. 204 Sartear, 3.0. 257 Sarkar, S.K. 154, 157 Sarma, K.S. 78 Sauna, P.B.K. 26 Sathlanaccan, K. 6) Satyanarayana, M. 255 Saxena, 3.D. 193 Saxena, R. 1)1 Sebastian, P.J* 6) 1 Selvanayagam, D.B. 255 Sen,D. 19, 233 Shah, R.T, 257 Shaima, 3.3. 201 Sharma, K.K. 126, 142 Shaima, S,D. 183 Shikaikhane, N.3. Singh, 5.3. Singh, J.F. Singh, R.D. Sivrara, 2.M, Srlvastava, 3.P. 3ood, D,K, Sunrisran, K.P. Thakur, K.B. Thaku.r, S.iT. Thattey, 3. T rip a th y , A.K. Tripatljy, A.K. IJppal, J.3i Varadarajan, T.3. Venkateswarlu, P. tQ ilko u i JiU snjiLny