Proceedings of the 6th International Symposium on Advanced Nuclear Energy Research
-INNOVATIVE LASER TECHNOLOGIES IN NUCLEAR ENERGY-
IMPACT of LASER and NUCLEAR TECHNOLOGIES V.S.LETOKHOV
Institute of Spectroscopy Russian Academy of Sciences Triotzk, Moscow Region, 142092, RUSSIA
A brief review is given of the applications of laser spectroscopy techniques and laser separation processes at an atomic-molecular level in nuclear physics and technology.
Keywords: laser spectroscopy, laser separation, isotopes, nuclear isomers.
1. INTRODUCTION
A series of principally new technologies have been developed over the 20th century that have formed the basis for a revolution in modem science and engineering. While lagging in its evolution some
30-40 years behind the nuclear technology, the laser technology has already reached a level allowing it to exert a noticeable effect on other fields of science and technology, nuclear physics and technology in particular. It should be emphasized in this respect that the laser technology is now at its exponential development stage still far from saturation. And so its application trends observed to exist today will be enhanced many times over in the years to come.
Table 1 generally lists the main trends in the application of lasers in the nuclear technology, the types of lasers needed for particular applications being indicated. The lecture briefly considers only the first two trends based on the resonance laser- matter interaction, which enable one to implement laser detection (and diagnostics) and laser separation at an atomic-molecular level. The applications based on nonresonance laser- matter interaction are equally important but they fall outside the scope of my interests.
The laser methods for detection and separation at an atomic- molecular level are potentially capable of being successfully used at all the stages of the nuclear fuel cycle. First, the laser detection of the traces of radioactive and dangerous chemical species contaminating the environment can be employed at the nuclear power station itself, at the fuel- reprocessing and chemical conversion plants,
-11- and at the radioactive waste storage sites. Secondly, laser diagnostic techniques can be used to monitor the state of the critical reactor elements and to prospect for uranium deposits. Thirdly, laser separation
techniques can be employed to achieve a more efficient separation of uranium isotopes, to produce
monoisotopic materials for some construction reactor components, and probably to extract radioactive
isotopes for other applications in the chemical reprocessing of radioactive wastes.
Table 1. LASER SCIENCE and TECHNOLOGY for NUCLEAR TECHNOLOGY
Laser Spectroscopy _>. Laser Tunable LasersLaser Detection Fiber Optics and Diagnostics
Resonant Interaction of Laser Separation Pulsed High Average Laser Light with Atoms (Isotopes,....) Power Tunable Lasers and Molecules
Laser High Power Pulsed Processing of and CW Lasers Materials Nonresonant Interaction of Laser Light with Substance Laser High Energy and Thermonuclear """" Peak Power Pulsed Fusion Lasers
2. LASER SPECTROSCOPY
Unique properties of laser radiation (monochromaticity, temporal and spatial coherence, high
power, controllable pulse duration) has made it possible to revolutionize all the characteristics of optical
spectroscopy. It has now become possible to attain the ultimate of these characteristics (Table 2). All
these characteristics are potentially important for nuclear physics and technology applications. I will
restrict myself to the discussion of the possibilities of implementing the laser spectroscopy for study of
atoms with short- lived nuclei in ground and excited metastable state (nuclear isomers), detection of
atoms with very rare long- lived isotopes and traces of radioactive atoms.
- 12- Table 2. LASER SPECTROSCOPY
CHRACTER1ST1CS UMJTATJONS 1. Spectral Resolution - Homogeneous Width, - Interaction Time with Laser Wave -103- 1 Hz 2. Temporal Resolution - Period of Light Oscillations - 10-14-10'15s 3. Sensitivity - Atomic- Molecular - Single Atom Structure Molecule 4. Selectivity - Overlapping of Wings - Very Rare of Spectral Lines Isotopes 10"10- 10-20 5. Spatial Resolution - Light Wavelength, -0.1- 1.0 mem - de-Broglie Wavelength - 1-10 A 6. Remoteness - Length of Propagation - Cross- section of Scattering -0.1- 100 km
Laser spectroscopic techniques are greatly diversified (see monograph [1]), but a unique position among them is held by the laser resonance ionization spectroscopy (RIS) [2, 3] which allows one to achieve ultimate characteristics, specifically very high sensitivities and selectivities in detecting and separating of atoms and molecules. Figure 1 presents a schematic diagram of a laser resonance ionization spectrometer used to study the proportion of short- lived nuclei ( mean- square charge radius variation A
This method has been used at the Konstantinov Institute of Nuclear Physics and the Institute of
Spectroscopy to investigate the isotopic shifts and hyperfine structures of the isotopes of Nd (A= 132,
134- 142), Sm (A= 138- 145, 147, 149, 150, 152, 154), Eu (A= 138-115), Ho (A= 152- 165), and Tm
(A= 156- 172) and other rare- earth elements [5]. These measurements have yielded the mean square charge radii of the nuclei of these isotopes, their electromagnetic moments (with the exception of the even-even isotopes and the Eu isotopes), improved values of the spins of certain nuclei, and the isotopic behavior of the charge radius A of rare earth elements forN< 82 and 88< N < 94, i.e., on either side of the magic number N= 82. Fig. 2 shows the values of A
Sm, and the previously investigated [6] isotopes of Ba, Cs, Xe (using a different system). As can be
- 13 — seen, A
From the standpoint of nuclear physics, the success of laser optical spectroscopy in the study,
detection, etc. of nuclei is based on two properties: (1) the large cross section for the resonance
excitation of optical transitions in the electron shell (a0pt= X^/ 2n= 10**0 Cm2= 10^ barn) and (2) the
high intensity of even relatively modest laser beams (1 W/cm^ corresponds to an intensity 1= 10^
photon/ cm^s). This ensures the high rate of resonance excitation Wext= o"0ptI= 10" s"* of atoms in a
beam for given nuclear charge Z and given neutron number N.
Fig. 1 General view on laser resonance ionization spectrometer of Leningrad Institute of Nuclear Physics and Institute of Spectroscopy for study of short- lived isotopes generated by lGeV proton beam from accelerator.
Moreover, the scope of laser methods is actually much more extensive: it is possible to achieve
not only isotopically, but also isomerically selective excitation of atoms, which means that isotopic and
isomeric nuclei can be detected and separated by laser radiation [7], Selective laser photoionization of
nuclear isomers was first observed in on- line experiments on the hyperfine structure and isotopic shift
of the atomic lines of the radioactive isotopes of europium [5a].
— 14 — A
Number of neutrons Number of neuirons a b
Fig. 2 Variations of mean- square charge radius A
The nuclear isomers of samarium- 141g, 141m, and thulium- 164g, 164m have been separated
for the first [8] time by selective laser photoionization of atoms in an on- line experiment, using a proton
accelerator and a mass separator of radioactive isotopes. The principle employed was similar to that
illustrated in Fig. 3. Photoions with an excited nucleus of 141mSm or 164Tm, produced in isomer-
selective three- step photoionization, were extracted by the electric field from the region of interaction
between the laser radiation and the atomic beam, and were deposited on the cathode of a secondary-
electron channel multiplier. Fig. 3 shows the photoionization spectrum of a mixture of the Sm
isomers (laser wavelength tuned to the first step) together with an interpretation of the results. It is clear
that the photoionization spectrum contains well- resolved ions belonging to 141mSm. This means that,
when the laser frequency corresponding to the first step is tuned in the range 1-4 GHz, a beam of
photoions containing isomeric nuclei is produced in the direction perpendicular to the atomic beam. This
system can be used to investigate isomers with half- lives of the order of the time necessary to liberate
them from the target (down to 1 s).
- 15 — Ground slate Excited state "Sm,. of the nucleus ol the nucleus 1-1/2 /- tt/2
3/2 6161,SA J226s,6c»f •J/2
6751,5% ir'esspfy -2 -! 0 1 2 J * 5 6 GHZ 6004,2% m+g || | g III 111 I I I I , 7 S 11 13 15 A III II V*6*%f 2Fj.j ' Z 2'T 2' 2 2 ml 1 III III II a b Fig. 3 Resonance ionization spectrum (a) for the first step (X]) of excitation of '41sm with the ground (g) and excited (m) nuclear states. The diagram (b) shows the position of the lines of HFS due to ^'Sm atoms with ground- state (g) and isomeric (m) nuclei (from [8]).
There is relatively large number of very rare long-lived radioactive isotopes of cosmogenic and technogenic origin. In principle, any of the techniques of laser spectroscopy that are capable of detecting single atoms can be used for the highly selective detection of rare isotopes. The basic difficulty here is the attainment of maximum detection selectivity S, i.e., the detection of a small number N^ of atoms a very rare isotope A in the presence of a much larger number Ng of the main isotopes B (S= N-QI N^).
Selectivity is due to the small isotopic shift Avjs= v^- VQ of the spectral line resulting from one or several successive resonance transitions in the atom from ground to excited states. The width of the spectral line is the natural limitation on selectivity because of the overlap between the wings of the closely- spaced spectral lines of atoms A and B. However, the nature of this limitation is significantly different for different methods.
The most promising methods are based on resonance multistep excitation of the rare isotope in a multifrequency laser field, using the isotopic shift of several successive resonance transitions. The net result is that the selectivities S^ at the successive excitation and ionization steps are multiplied together.
Practical implementation of the principle of multiplication of selectivities encounters the difficulty that, for the most interesting long- lived isotopes, it is difficult to find an upward sequence of transitions with appreciable isotopic shifts, since only the ground state of the atom has an appreciable shift.
A universal way of overcoming this difficulty, and transforming the method of stepwise ionization into a real method of detection of rare isotopes, was proposed in [9]. The idea is to use collinear stepwise photoionization of a beam of accelerated atoms. The ions are accelerated and
-16- neutralized, and this is accompanied by the bunching of the longitudinal velocities of the atoms and, hence, by the removal of Doppler broadening during collinear excitation. The reduction in Doppler width is, in turn, accompanied by a Doppler shift of all the spectral transitions in accelerated atoms, which depends on the ion mass. This results in an artificial kinetic isotopic "mass" shift in all atomic transitions.
The method suggested in [9] was successfully implemented in a number of works. For example, a selectivity of 10^ was attained at the Institute of Spectroscopy in detecting the rare isotope 3 He
against the background of the abundant isotope ^He [10]. Based on this methods, an experimental setup was developed at the Mainz University for detecting the rare isotopes ^Sr and ^^Sr. Strontium- 90 is a most dangerous isotope, for it can accumulate in human bones and produce a high local radiation dose.
The detection limit attained amounted to 50- 10^ ^Sr atoms in the presence of 10*8 stable strontium
atoms in environmental samples [11]. So high a selectivity was achieved by combining preliminary mass
separation (selectivity over 1(P) with collinear resonance ionization.
To monitor the radioactive contamination of the environment specifically to reveal spreading
pathways and accumulation sites (those in the human body included), one has to detect radioactive
atoms at a very high sensitivity not attainable with the existing dosimeters. The RIS technique enables
one to do this. For example, the authors of [12] have managed not only to detect the ^21pr atom
(lifetime 22 min), but also to measure its optical spectrum, the number of the atoms in the sample being
a mere 105-106.
Plutonium is one of the most serious radioactive poisons in the environment, originating from
nuclear bomb test, nuclear power plants, and different type nuclear accidents in particularly Chernobyl
accident. Ultrasensitive and selective methods are required to study the ecological behavior and
migration in the environment. The detection technique actually used is a- spectroscopy, which has some
severe drawbacks: the sensitivity is limited (4- 10^ isotopes for "9pm)( fa& e]ement and isotope
selectivity is restricted because of other isotopes emit a- particles with similar energies, the
measurements take a long time, and 241pu can not be detected at all because of the absence of a-
emission.
-17- RIS method is an alternative, promising method. Its detection limit does not depend on the nuclear properties of the isotope under investigation such as the half- life and decay mode. The detection of trace amounts of Pu in environment samples was effected in [13] by the RIS technique used in combination with mass spectrometry (the RIS technique [2]). The sensitivity of detecting 239pu jn this experiment was as low as 10? atoms.
3. LASER SEPARATION
Tunable lasers made it possible selectively to excite practically any single quantum state of an atom or a molecule in the range 0.1- 10 eV. Systematic studies of the resonant interaction between laser radiation and matter have been under way since around 1970, mainly with a view to developing the laser isotope separation (LIS) process. Today, almost 25 years after the studies were commenced (see the review of these works in the monograph [14]), it has become clear that there exist at least three economically viable LIS methods (Fig. 4):
(1) the multistep isotope- selective photoionization of atoms, suggested in [15, 16];
(2) the two- step IR- UV isotope- selective photodissociation of molecules, proposed in [16,
17]; and
(3) the multiphoton IR isotope- selective photodissociation of polyatomic molecules, discovered in [18, 19].
fatf/Mfff A++ e" n-l+B h(D2
/iffli
a) b) c) Fig. 4 Economically viable methods of laser isotope separation: a) multi- step isotopically selective ionization of atoms; b) two- step IR-UV (vibrationally- mediated) isotopically selective photodissociation of molecules; c) multiple- photon IR isotopically- selective photodissociation of polyatomic molecules.
The multistep isotope- selective photoionization of atoms was successfully used in the
Lawrence Livermore Laboratory Program AVLIS for separating uranium isotopes [20J. The IR- UV
-18- isotope- selective photodissociation of molecules (UF5) was also studied for the purpose of uranium isotope separation at the Los- Alamos Laboratory [21]. And finally, the multiphoton (MP) IR isotope- selective photodissociation of polyatomic molecules is now at the stage of practical application to separation of isotopes of both light elements (e.g. ^C and ^C [22]) and uranium [23].
Since its decision in 1985 to forego continued development of gas centrifuge technology, the
Department of Energy of USA has spent hundreds of millions of dollars developing AVLIS technology to enrich uranium for commercial- grade nuclear fuel. The major AVLIS plant systems and facilities are
[24]:
(L) the chemical processing of uranium compounds to produce the metal feed;
(2) die process of dye lasers that generate red- orange light used to photoionize (by three- step excitation) the heated uranium vapor;
(3) the copper- vapor lasers that pump the process lasers,;
(4) the separators for enrichment of uranium in the 235U isotope;
(5) the chemical processing to convert the metal product to uranium oxide.
The AVLIS laser system (copper lasers with average power «10 kW and dye tunable lasers with average power »2 kW) has been exceptionally well engineered at LLNL. It is major advance in laser engineering.
The AVLIS technique is fairly universal and can be used to separate the isotopes of a series of elements., although a special R/D program is required for each element. At least three examples of
Applications of AVLIS technology can be given: Gd, Pu, Zr [24].
(1) The naturally occurring mixture of gadolinium oxides is now as turnable poison in essentially all building water reactors and many pressurized water reactors. Greater improvement with
Gd poison certainly could be achieved of ^Gd enriched material were available, particularly for fuel elements. Availability of Gd enriched to 80% ^^Gd would mean that Gd loading in the fuel could be reduced.
(2) Pu isotopes (238, 239, 242, 244) are unique and invaluable for powers sources, safeguards, and other special applications. The quantities required are tens or hundreds of kG for 2"Pu, tens of kG
-19- 2 2 for ^ Pu, and the order of lg for 244pu Production of these valuable isotopes is quite possible by
AVLIS.
(3) Most of hundreds power reactors in the world use 235TJ. enriched uranium oxide pollets clad in low- hafnium zirconium. All of these reactors pay heavy penalties with regard for performance
and economics because of neutron absorption by the "'Zr isotope and residual hafnium. AVLIS offers
the potential for removing this isotope and hafnium.
Let us finally emphasize that die multistep resonance ionization of atoms makes it possible to
separate not only elements (A- selectivity) and isotopes (A, N- selectivity), but also isobars where A=
Z+ N, A, Z, and N being the number of nucleus, protons, and neutrons in the nucleus, respectively, and
E is the isomeric nucleus excitation energy.
The molecular laser isotope separation (MLIS) technique is very an alternative of the AVLIS
technique. The MLIS technique is universal and applicable to all atoms that can be incorporate in
volatile molecular compounds, because isotope shifts inevitably manifest themselves in the vibrational
spectra of the molecules. Especially efficient is the MLIS technique based on the IR MP isotope-
selective photodissociation of molecules [18, 19].
Many experiments on isotopical selectivity of MP molecular dissociation have dius been
performed. These experiments have covered many isotopes, from light ones (hydrogen, deuterium,
tritium) to heavy ones (osmium, uranium) contained in very different molecules [22, 26]. Many of these
experiments became the basis of the laser isotope separation metiiods developing in numerous
laboratories of several countries. As an example, let us present more detailed data for the case of "C
isotope separation which was under development in the USSR through the cooperation of several
Institutes. (Institute of Spectroscopy, Kurchatov Institute of Atomic Energy and Institute of Stable
Isotopes).
To developed an economically viable method of isotope separation it is necessary first of all to
choose a polyatomic molecule which satisfies many requirements simultaneously: 1) high yield of MP
dissociation for a laser pulse energy fluence which is acceptable for optical windows of laser separation
cells (less than 2- 3 J/cm2); 2) high isotopical selectivity of MP dissociation for irradiation at CO2 laser
— 20- wavelengths; 3) low cost of initial molecular compound. In a present study of a large number of polyatomic molecules it was found that the molecule CF2HCI (Freon- 22) is optimal.
For reliable operation of the laser separation module it is very important to reduce the
requirements of the energy of the laser pulse performing the efficient IR MP dissociation. In this case,
first of all we can use the nonfocused beam of the CO2 laser and irradiate the molecular gas mixture in
a long separation call. Secondly, the probability of laser damage to the optical windows of the
separation cell can be diminished. The effective method of reaction where the first laser pulse performs
the isotopically- selective excitation of molecules in the vibrational quasicontinuum up to the
dissociation limit [26]. Two- frequency irradiation of the molecule CF2HCI improves the parameters of
carbon isotope separation significantly. Multifrequency irradiation of this model gives much better
results [27]. In this case very high selectivity of MP IR dissociation (up to 10^) and high yield of
dissociation can be achieved simultaneously. Figure 5 presents the dependencies of dissociation yield of
the CF2HCI molecule on the energy fiuence of the CO2 laser pulse in cases of 2- frequency, 3-
frequency and 4- frequency irradiation. Points of required degree of selectivity (100) of MP IR
dissociation are marked.
100 ~I TTTTT1 1 1 i 1 i"i 11| 1 1 1 1 1 rrr
CF2HCI ^
UJ >- S»IOO 2: 10 O < o 1050 ijQo o WAVENUMBER.CM-' to S2 o ' I t I 11 III B. O.OI 0.1 I 10 FLUENCE, J-CM-2
Fig. 5 Dependences of the dissociation yield for the CF2HCI molecule on the fiuence of the first laser pulse in the cases of (a) double-, (b) triple-, and (c) quadruple- frequency excitation. The arrows indicate the values of yield obtained at a selectivity of 10^ in the case of double-, triple-, and quadruple- frequency MP dissociation. Positions of laser frequencies for all cases are indicated on the linear IR absorption spectrum of CF2HCI (upper curve) (from [27]).
-21- This technique can also be used to separate other isotopes of interest in the nuclear technology, uranium isotopes in particular, by way of the IR MP dissociation of the UF6 molecule. For isotopes of heavy elements, vibrational isotope shift is relatively small, and so use should be made of gasdynamically cooled molecular beams in conduction with multiple- frequency IR excitation. It is also important to know the specific characteristics of the molecules in both low- and high- lying vibrational- rotational states. Table 3 lists the IR MP excitation parameters of polyatomic molecules that are critical to the optimum MLIS process.
Table 3. CRUCIAL (for MLIS) PARAMETERS of IR MPE/D of POLYATOMIC MOLECULES
PARAMETER PESULT
1. LOW-LYING RESONANT OPTIMAL FREQUENCIES for HIGH VIBRATIONAL- ISOTOPICAL SELECTIVITY of IR ROTATIONAL TRANSITIONS EXCITATION 2. VIBRATIONAL NUMBER of RESONANT VIBRATIONAL STOCHASTIZATION ENERGY EXCITATION STEPS ONSET 3. ABSORPTION SPECTRUM of OPTIMAL FREQUENCY and MINIMAL HIGHLY- ENERGY FLUENCE of IR VIBRATIONALLY EXCITED DISSOCIATING PULSE STATES
The most advanced MLIS program for uranium is under way at the AEC of South Africa [23].
Finally, we believe it quite possible to use laser selective photoprocesses for the processing of
nuclear wastes, specifically to (a) extract uranium and plutonium from liquid wastes, (b) return long-
lived fission products to the reactor for their nuclear transmutation to stable, nonradioactive isotopes,
(c) extract heat- generating radioactive elements, and (d) extract valuable metals, such as rhodium and
4. CONCLUSION
(1) Laser control of nonproliferation of nuclear weapon using ultrasensitive and ultraselective
laser analytical methods;
(2) y- laser using laser separation of nuclear isomers [28] and laser search and study of
appropriate excited metastable nuclei [4- 6].
— 22 — REFERENCES
1) Demtroder W: Laser Spectroscopy (Springer- Verlag, Berlin) 1981
2) Letokhov V.S.: Laser Photoionization Spectroscopy (Academic Press, Orando) 1987
3) Hurst G.S. and Payne M.G.: Principles and Applications of Resonance Ionization
Spectroscopy (Adam Hilger, Bristol) 1988
4) Otten E.W. Investigation of Short- Lived Isotopes by Laser Spectroscopy (Harwood Acad.
Publ., Chur) 1989
5) a) Fedoseev V.N., Letokhov V.S., Mishin V.I. et al.: Optics Comm, 52, 24 (1984)
b) Zherikhin A.N., Kompanets O.N., Letokhov V.S. et al.: Zh. Eksp. Theor. Fiz. 86, 1249
(1984); c) Mishin V.I., Sekatskii S.K. et al. Zh. Eksp. Theor. Fiz. 93, 410 (1987); d) Alkhazov
G.D., Barzakh A.E., et al. Pis'ma ZhETF., 46, 136 (1987); e) Nuclear Physics, A477. 37
(1988); f) ibid, 337 (1990); j) Letokhov V.S., Mishin V.I. et al., J. Phys. G.: Nucl. Part. Phys.,
18,1177(1992)
6) Otten E.W.: Treatise on Heavy- Ion Science, vol. 8, ed. by D.A. Bromley (Plenum; New
York) p. 517 (1989)
7) Letokhov V.S.: Optics Comm., 2, 59 (1973)
8) Mishin V.I., Sekatskii S.K., Fedoseyev V.N. et al.: Optics Comm., 61., 383 (1987)
9) Kudriavtsev Yu.A. and Letokhov V.S. Appl. Phys. B29, 219 (1982)
10) Aseyev S.A., Kudriavtsev Yu.A., Letokhov V.S. and Petrunin V.V. Optics Lett., 16, 514
(1991)
11) Monz L., Hohmann R., Kluge H.-J. et al. Resonance Ionization Spectroscopy -1992, ed. by
CM. Miller and J.E. Parks (IOPP, Bristol) p. 225 (1992)
12) Andreev S.V., Letokhov V.S., and Mishin V.I. Phys. Rev. Lett., 59, 1274 (1987)
13) Urban F.- J., Deisenberger R., Herrmann G. et al. Resonance Ionization Spectroscopy- 1992,
ed. by CM. Miller and J.E. Parks (IOPP, Bristol) p. 233 (1992)
-23- 14) Letokhov V.S.: Nonlinear Laser Chemistry. Multiple Photon Exciation, (Springer, Berlin) p.
417(1983)
15) Letokhov V.S.: Soviet Patent No 784679. Appl. on March 30 (1970)
16) Ambartzumian R.V. and Letokhov V.S.: Appl. Opt. Ji, 354 (1972)
17) Letokhov V.S.: Soviet Patent No 784680. Appl. on March 30 (1970)
18) Ambartzumian R.V., Letokhov VS., Ryabov E.A. and Chekalin N.V.: Pis'ma ZhETF, 20, 597
(1974)
19) Ambartzumian R.V., Gorokhov Yu.A., Letokhov V.S. and Makarov G.N.: Pis'ma ZhETF, JI,
375 (1975) [JETP Lett. 21, 171 (1975)]
20) Paisner J.: Appl. Phys. 43B, 252 (1988)
21) Jensen R.J., Judd O'D.P. and Sullivan J.A.: Los Alamos Science, 3, No 1, 6 (1982)
22) Velikhov E.P., Baranov V.Yu., Letokhov V.S., Ryabov E.A. and Starostin A.N.: Pulsed CO-
Lasers and their Application for Isotope Separation (Publ. House "Nauka", Moscow) pp.
304 (in Russian) (1983)
23) Du Toit G.: The South African Mechanical Engineering. 42, 61, (Febr. 1992). Kemp D.M,
Bredell P.J., Ponelis A.A.and Ronander E. Bull, of the Research Lab. for Nuclear Research.
Spec. Issue l,p. 1 (1992)
24) Alternative Applications ofAVLIS Technology. Report. (Nat. Acad. Press, Washington D.C.)
(1991)
25) Alkhazov G.D., Barzakh A.E., Denisov V.P. et al. Nucl. Instr. and Meth. in Physics Research.
B69. 517 (1992); Mishin V.I., Fedoseyev V.N., Kluge H.-J., Letokhov V.S. et al. ibid., B73.
550(1993)
26) Bagratashvili V.N., Letokhov V.S., Makarov A.A.: Multiple Photon Infrared Laser
Photophysics and Photochemistry. (Harwood, London) p. 512 (1985)
27) Evseev A.V., Letokhov V.S., and Puretzky A.A: Appl. Phys. B3JL 93 (1985)
28) Letokhov V.S.: Zh. Eksp. Theor. Fiz. 64, 1555 (1973).
— 24 —