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JOURNAL OF RESEARCH of the National Bureau of Standards-A. Physics and Chemistry Vol. 68A, No.4, July-August 1964

Mass Spectrometric Study of Photoionization. 1. Apparatus

and Initial Observations on Acetylene, Acetylene-d2f

Benzene, and -d6*

Vernon H. Dibeler and Robert M. Reese

(April 9, 1964) A windowless vacuum monochromator and mass spectrometer are combined for the study of photoionization proc('sses in the energy range 2000 to 600 A (6 to 21 eV). Details of t hc apparatus and techniques of operation arr given and results are reported for an initial study of acctylene, acetylene-d2, b enzene, and benzene-d6. energies of 11.406 and 11.416 eV are obtained for the l".u electron of C 2H 2 and C 2D 2, respectively. Vibrational levels of th e ground state of the ion are observed wi t h quantum intervals of 1855 crn- I (C2H 2) and 1775 Cm.- I (C2D 2) . Ionization energies for the el g ( "') electron of C6H6 and C6D6 are determined to be 9.242 and 9.245 eV, respectively. Quantum. intervals for vibrational levels of the ground state ions are apparently equal for the t wo isotopic molec ules and estimated to be 800 cm- I • A second onset of ionization is observed at 11.53 eV for C6H 6 and at ll.5, eV for C6D 6. R esults agree well with spectroscopic data.

1. Introduction

K early monochromatic beams ha\re been A combined mass spectrometer-vacuum ultraviolet used for many years [1]1 to study photoionization monochromator designed to avoid the above diffi­ processes in low-pressure gaseous systcms. The culties was recently completed and is being applied recent addition of mass spectrometric techniques to the study of photoionization processes in the [2,3] to identify unequivocally the ions produced in energy range 2000 to 600 A (6 to 21 eV). Details the photoionization process has obviated any uncer­ of the arrangement, the method of operation and the tainty in the chemical composition and has shown results of some observations on acetylene and ben­ that, at least in principle, considerable information zene are given in the following sections.2 can b e obtained on the upper energy states of poly­ atomic and the dissociation processes of 2 . Experimental Details ions [4]. In surmounting the experimental difficulties of Measurements were made by means of the appa­ combining vacuum ultraviolet spec troscopy and ratus shown schematically in figure 1. A photon , Hurzeler et :11. [2] used a source (L ), a vacuum ultraviolet monochromator fluoride window to isolate a -discharge OJ;J), and a mass spectrometer (N) are combined photon source from the high-vacuum regions of essentially in the manner first described by Hurzeler the monochromator and mass spectrometer. Thus, et a1. [2]. An important exception, however, is the although the photon source included a broad energy complete absence of absorbing windows to isolate range, the absorption of the window limited th~ high pressure regions from the monochromator and measurements to wavelengths longer than 1040 A mass spectrometer. Thus, the entire vacuum ultra­ (E<1l.9 eV). violet region is accessible. Weissler et a1. [3] used no windows in their appa­ ratus but cmployed a repetitive low-pressure spark 2 .1. Monochromator to produce a widely spaced line spectrum. For some of the molecules studied, the lack of a sufficient num­ The Seya-N amioka type [5] instrument with a I-m ber of emission lines in the vicinity of an onset of focal length was built to specifications by the Mc­ ionization prevented establishing that onset to better Pherson Instrument Co., Acton, Mass. The photon than ± 30 A (± 0. 5 eV). Also, the presence in the exit arm to which the mass spectrometer is appended mass spectrometer of background gases from the is nonmagnetic stainless steel. Usual materials are photon source resulted in undesirable effects on ion used elsewhere. Three optical entrance slits, sized form:1tion. 50, 100, and 500 fJ. , are mounted on a turret located at SI (fig. 1) which can be rotated from outside the vacuum. ' This wo rk was supported in part by Project Derender or Advanced Research Projects Agency, Department of Defense and by the Atomic Energy COJl':lnission. 1 Figures in brackets indicate the lIterature refereDces at the end of thIS paper. 2 A brief annonncemen t of this work appears in J. Chem. Phys. 40, 2034 (1964). 409 2.2. Mass Spectrometer Analysis and detection of the photoions is accomp­ lished by means of a conventional 15-cm radius of curvature, single focusing, sector-field instrument fabricated of Inconel and stainless steel. All vacuum seals are gold wire O-rings. The ion-source housing and analyser tube are separated by the ion defining slit (85 ) and are separately pumped through 5-cm gate valves, by liquid traps and mercury diffusion pumps. The untrapped speed of the diffusion pumps is reported by the manufacturer to be 50 l /s at 10- 3 torr. Under conditions indicated above, the pressure differentials between lamp and and between lamp and analyser tube are FIGU RE 1. Schematic diagram showing the arrangement of of the order of 10 8 and 10 9, respectively. Thus with photon source (L) , monochromator (M ), mass spectrometer a lamp pressure of 400 torr, the pressure in the (N), slits (S), and pumping ports (P ) . analyser tube of the mass spectrometer is about S L, photon source slit. 0.3X6 mill; S" S" entrance and exit optical slits O.I X IO 4 X 10- 7 torr. ~ ; 82, ,83, d~fferent ial pumping slits, 2X4 nun and 4X5 1l1Jl1, respe cti~el y; 851 lon definlllg slIt, 0.5X IO m111. P L, 8 1/s mechanical PWllP; P , 400 I/s H eraeus­ The ion-source housing and analyser tube are ll;0ots! 80 Ifs forepulllp; p " 700 lis .oil diffusion, 80 I/s forepw,; p; P 3, 1400 I/s oil diffusIOn , 40 I/s forcpl1mp; P" P" I!q. N, t raps, JIg diffusion and forcpl1ll1ps. supported by the photon exit arm of the monochro­ mator. Locating pins in the vacuum fianges main­ tain appropriate alinement of the photon beam, ion The concave, triparti~e grating (Gr) supplied by source, and analyser. The optical exit-slit mount is Bausch and Lomb contams 600 grooves per mm in a electrically insulated from the monochromator. The m led area 56 X 96 mm. It is blazed for 1500 A and slit assembly is attached to the ionization chamber ~oated with llmgnesium fluoride. With the grating by screws thereby forming one wall of the chamber. ill place, the first-order dispersion of the mono­ The slit assembly (and ionization chamber) is <6hromator is stated by the manufacturer to be 16.6 attached to the mount by means of a self-locating A /m~. The gr~ting is rotated manually or at any dovetail connection. Interchangeable slit assem­ one of 12 scannlllg speeds by a synchronous motor. blies to match the entrance optical slits are available. The wavelength is read from a counter coupled di­ Figme 2 is a photograph of the detached ion r ~ ctly ,to the gr~ting dri.ve. .Adjustments are pro­ source showing the ionization chamber and the ion Vided for correctmg the Imeanty and the calibration accelerating and focusing plates. The dovetailed of the scale. Wa¥elengths can be selected manually exit optical-slit assembly forming one wall of the ioni­ to better than 1 A when traveling in one direction. zation chamber is visible in the right foreground. The monochromator is provided with two differ­ The sample inlet tube and a filament assembly and ential pumping ports (p !) P 2) separated by "lits (8 2 anode used for electron impact measurements are S3) , and located immediately behind the entranc~ removed for the photograph. optical sli t (8 [ ). PI is evacuated by a 400 lis (at Figure 3 shows the ion-source housing with the 1 torr)3 Heraeus-Roots pump; P 2 , by a 700 lis (at cover plate removed and the source mounted in 10- 3 torr) oil-d~ffusion pUJ:J?-p. Each is backed by an place. The direction of the photon beam is toward 80 lis (free all') mechamcal pump. The grating the observer and approximately perpendicular to chamber is evacuated (P3) by a 1400 lis (at 10- 3 the plane of the photograph. The large slit in the torr) oil-diffusion pum.p backed by a 40 lis mechani­ center permits the diverging photon beam to leave cal pump. Freon or -cooled baffles, suitable the ionization chamber without striking metal gate valves, and vacuum gages are provided at each surfaces until it reaches the photon monitor mounted port. Seals in high-vacuum regions are made with on the coverplat.e. The photoions are accelerated O-rings fabricated of fluorocarbon elastomer. Other out of the ionization chamber and toward the right O-rings are made of Buna-N. of the figure. The chosen geometry of the system The basp pressure of the monochromator is about (orientation of photon slit relative to ion slits) re­ 3 X 10- 7 torr. With hydrogen at a nominal pressure sults in ease of construction and alinement. Un­ of 10 torr flowing into G[ and with slit sizes as indi­ fortunately, it also results in a smaller usable cated in figure 1, the system maintains a pressure ionization volume compared with that in the geom­ differential of 105 between photon lamp and mono­ etry used by Hurzeler et al. [2]. c~romator chamber. With the lamp operated at Gas samples are admitted to the ionization cham­ higher pressures (> 100 torr), additional pumpino. is ber by means of an all-metal gas handling system, provided at P L by means of an 8 lis (free ~ir) a 2-liter reservoir, and gold pinhole leak. Mass mechanical pump. This results in a pressure analysis is performed at a constant ion acceh'rating differential of 106 between lamp and monochromator. voltage of 2 kV by varying the magnetic field. The latter, set to focus a selected ion at the collector, is ' 1 torr-l/760 atm·133.32 ncwtons-ll1eter-' (N/m'). monitored by means of a Bell model 240 incre-

410 ENERGY, ev 9.0 10.0 11.0 120 13.0 14.0 LYMAN a 1216 a HYDROGEN

LY MAN /3 1026 a

1500 1400 1300 1200 1100 1000 900 F I G U RE 2. Ionization chamber and ion accelerating plates. WAVELENGTH, a 'rhe photon slit in the wall o[ t he ionization chamber is just visible in the FIGUBE dovetailed mount. r:t.,wo threaded rods arc the ion~rcpcllcr co nnections. 4. T racing of the recorded photoelectric current fj'om the photon detector as a f1tnction of wavelength fo1' a d-c hydrogen discha1'ge at a p1'eSSUl'e of a few to rr. Entrance and slits are 100" and the scan rate is 100 A/l11in.

2.3, Photon Source Two types of lamps are used as photon sources. The first is a Hintereggel' type similar to that de­ scribed by Huffman, Tanaka, and Larrabee [6]. 1n operation, highest commercial puri ty cylinder gases without further purification enter through 0 1 and exit through the differential pumping ports. The di scharge is excited by charging energy storage capacitors from an unregulated high-voltage d-c supply through a curren t-limiting resistor. For hydrogen at a pressure or a few torr, a st.eady operating current of 250 mA is obtained with about GOO V across the lamp. Figure 4 is a tracing of the phoLoclectric current [rom the photon detector as a function of the wavelength. The spectrum is scanned at the rate of 100 A/min. Entrance and exiL optical slits ~lre 100 J..t and no correction is made [or the photoeletric efficiency of the detector. The Lyman-a line is easily iden tifiecl and the rrla­ tively broad energy range of 1500 to 900 A ( to FIGURE 3. Ion-source housing with cover removed to show the 14 eV) shown here indicates the general usefulness mounted ionization chambej' and ion accelerating plates. of this source of radiation. However, the large and The photon beam lcaves the ion chamber by tbe large rectangular slit and strikes the detector mounted on the eover plate. Pbotoions are accelerated frequent changes in photon intensity with wave­ toward tbe ri ght o[ the figure. Tbe pwnping port at the bottom o[ the hODsin g length characteristic of the many-lines emission and the gas-inlet t ube at the top are visible. In usc, a fl exible Tellon tube con­ nects Lh e gas·inlet tube with the ionization chamber. spectrum superimposed on the Lyman continuum is a definite disadvantage. ContrfLry to the above, the well-known rare-gas continua [7, 8, 9] provide comparatively uniform intensity with wavelength, although over shorter mental gauss me tel' n,nd recorded by a strip-chn,rt energy range. A recent study [6] has given details recorder. Ion currents are measured with an elec­ of best conditions for producing the helium con­ tron multiplier, n,mplifier, and scaler. Maximum tinuum using a d-c discharge. ion currents are of the order of 100 counts/s or T anaka and Zelikoff [10] first used microwave 10- 17 A. ' Vi thin 20 A ("-' 0.2 eV ) below threshold, excitation in a windowless tube to obtain an emission maXlLllUU1. background currents are of the order of continuum in xenon. Subsequent work [11] appeared 1 count/so so promising that a windowless microwave source 411 - -,---- I ~ -.------

EN ERGY, eV 9.0 95 100 10 5 110 11. 5 12 .0 iI 1046'A_ - ]

ARGON -

H

N

-

f- Z w ~ a: a: - ::0 u u a: f- u w 1065'A-A +-1045 /l -' 10 47'A~ w 0 f- 0 I a. 14 00 1300 1200 11 00 - WAVELE NGTH, &

FIG URE 5. Tlacing of the 1·ecorded photoeled n c current of the photon momtor as a Junetton of wavelength f01' the argon _ 1064.8. - continuu m at 400 torr. I0 66 fJ,~ Slits are 100" and the scan rate is 100 ,," /mill. - was constructed for the presen t research. A 13-mm o.d. Vy~or tube \~itl.l l -11un wall~ was sealed by means of epoxy reSll1 ll1to a hole dnlled throuo'h the 10 4 8)\ _ I center of a water-cooled aluminum-alloy flange. ) The flanged ~nd of th~ tub~ was terminated by a C 0.3X 6-mm sitt m a thm stamless-steel sheet. The lamp was moun ted with the slit end a few milli­ WA VEL ENGTH , )\ meters in front of the entrance optical slit of the FIGU RE 6. Tmcing oJ the photoelectric CWTent of the photon monochromator. vVith argon flowing through the monitor as a f 1lnct 1: on oJ wavelength Jor the argon d01,bl et lamp at about 400 torr, the discharge is excited by resonance lines at low pressure (a f ew t01'1·) . a Raytheon model PGM- IOO , 2450 MHz, 250 to 'l'hc slits arc 100" and the scan rate is 10 l /lllin. The wavelength scale is un­ 800 vV, cw generator. Figure 5 shows a tracino' corrected. of the continuum recorded for 100-J.L slits at a rat~ of 100 A/min. The useful range in the present 2.4. Photon Detector experiments extends from 1400 to 1070 A (8.8 to 11. 6 eV). Small impurity peaks are us ually 0 b­ Initial measurements were made using a sodium ser:red ~md serve as convenient fiducial points for salicylate-coated photomultiplier tube [12) . How­ cahbratlOn of the wavelength scale. The stability ever, the quantum efficiency of the coating was of the lamp as measured by the photon intensity apparently affected by exposure to at any point on the continuum was usually better samples or residual pumping and it was than ± 1 percent at least during the time required discarded in favor of a photoelectric detector. The to obtain the ion count. cathode of the latter was made from a tungsten 4 Figure 6 shows a tracing of the argon doublet sheet 10 X 40 X O.l mm taken from the laboratory resonance lines at 1066.7 and 1048.2 A obtained at stock and cleaned chemically. The anode was low pressure (a few torr) to check the resolvinO' several O.l-mm Nichrome wires mounted close to power of the monochromator. The lines are scanned the surface and biased + 45 V with respect to the cathode. The photoelectrons leaving the cathode at a rate of 10 A/min using 100-J.L slits. The apparent were measured by means of a vibrating reed electrom­ half-width of the lines is about 2 A which is consist­ eter and recording potentiometer. The detector ent with a resolving power of l.7 A calculated from the slit dimensions. The actual line width of I In preliminary experiments, cathodes were made of a vari ety of metals in­ cluding rhenium, platinum, nichrome, and gold. K 0 significant advan tages course, is luuch narrower. ' over tungsten were observed. 412 was enclosed in an open-mesh grid to shi eld it from 2 .6. Materials the ion accelerating field in the ion source. N ever­ theless, there remained the possibility of accelerating Ordinary acetylene, with a purity of 99.6 percent, photoelectrons into the ionization chamber. This as stated by the supplier, was obtained from was shown to b e of negligible importance by changing the Matheson Co mpany. Ordinary benzene was the field penetration, and introducing a uxiliary research-grade material from the laboratory stock. electric and magnetic defle cting fi elds. Acetylene-d 2 and benzene-d6 were obtained from Although the quantum efficiency of photoelectron Merck , Sharpe, and Dohme, :Montreal and h ad a emitters varies with the w

350 c .2 o L 0. ~ 300 2 ci -1 13 42 'A 1327 'A W )- 250 9.2 4 ev, ,9.34 ev z o f-

100 _...... 50 - .,-....

O L-~'~"~"~"_'-____~ ____~ ____~ ____~ ____~ ____-L ____ -L ____ -L __ __-L ____ ~~ __~~ __~~ __~ 1370 1350 1325 1300 1275 1250 1225 1200 1175 1150 1125 1100 1075 1050 1025 WAVELENGTH , 'A

FIG UHE 9. a, b. Ionization effi ciency curves for the CoH o+ ion of benzene and the CoDo+ ion of benzene-do obtained by the use of the argon continuum and the hydrogen emission spectrum. Threshold energies for tbe gro wld'statc ions are indicated.

R ydberg series in benzene were first analyzed by electron from an (a2U) 1T- electron . Price and Wood [28] . More r ecently, Wilkinso n The general features of an ab orption curve of [29 ] obtained high-resolution absorption spectra of C6H6 reported by Goto [31] also show similarities bemr.ene and benzene-d6 and identified foul' R ydberg with the present results. serics all co n verging to the same ionization limits: From the second ionization potential (11.5 eV ) 9.247 ± 0.002 eV (C6H 6) and 9.251 ± 0.002 eV and the usual Rydberg equation, series m embers (CeD 6) ' These were assigned to the ionization leading to that ionization pootential are calculated: of the lowest energy 7r electron. The present one to appeal' at about 1175 A (10.5 eV) and the other measurem ents arc in good agreemen t with these to appeal' very dose to the first ionization limit. and with recent·, photoionization measurements Both members have been observed by Price and made without mass analysis [29] . Wood [28]. Wilkinson [29] r eports a strong absorp- The genera1 shapes of both curves are quite similar. tion doublet for benzene (1342.5 A and 1M1.5 A) A R ydberg term with well-defined vibrational and for benzene-cl6 (1335.4 A and 1338.2 A) which structure isoobsel'ved in t he r egion of 1180 A (C6H 6) are very close to the onset of ionization. As no and 1175 A (C6D 6) . Thc "peak" shapes indicate p eaks are observed at the onset of autoionization 0 1' pred issociation from these levels. ionization in the present work, it may be co ncluded A less well-defif),ed term or terms appears in the that these transitions are to a neutral region of 1120 A l eading to the second ionizatio1,1 rather than to the molec ule ion. tln'eshold at 1075 A (1l.53 eV) for C6H6 and 1070 A Electron impact studies of benzene [32, 33] have (1l.59 eV) for C6D 6. These observations ar e gen­ given indications of several electronic states above erally consistent with t he r ecent m easurements by the ground state of the ion. At present, the only Tanaka and coworkers [30] and the suggested fu'mly established onset would appear to be that assignment of the second ionization energy to an of tile (a2·,J 7r- electron at 11.5 e V. 415 ENER GY, eV 9. 25 950 9.75 10.00 10.2 5 10.50 10.75 11 .00 11. 25 11. 50 11.75 12.00 80 b + '70 C6 D6

60 c 1340 1\ 1326 1\ 1311 11 0 9.25eV 9.35eV 9.45 ev '0 , , , L 0. " 50 '"c .'2 0 ....J W :;::: 40 ...... z ..... ~ >-

0 .~ >- 0 .. - I 20 Q...... - .. - .... ,'"., 10 .. ' .' .~

. .... , 1370 135\) 1325 1300 1275 1250 1225 1200 1175 11 50 1125 1100 1075 1050 1025 WAVE LENGTH. 1\

FIGU RE 9- Continued.

~ n------'------'------'---, We acknowledge the advice and assistance of W. R. Shields, K. E. McCulloh, and M. E. Wacks in construction and maintenance of the apparatus. 13401\ 13261\ 131 0 1\ We are also indebted to M. Krauss and P. G. 9.25eV 9.35 eV 9.4 6eV c 2- C6 D6+ , , , Wilkinson for helpful discussions of results and to 0 .c H. M. Rosenstock and F. L. Mohler for encourage­ "- ~ ment throughout the course of the work. "c 20 0 0 4. References -'w ;: z [1] C. L. Weissler, Handbuch del' Physik, Springer-Verlag, '2 >- .... Berlin, 1956, Vol. 21, p. 304. - [3] C. L. Weissier, J. A. R. Samson, M. Ogawa, and C. R. 0 I Cook, J. Opt. Soc. Am. 49, 338 (1959). a. [4] W . A. Chupka, J. Chem. Phys. 30, 191 (1959). [5] M. Seya, Sci. of Light 2, 8 (1952) ; T. Namioka, ibid. 3, . 15 (1954) . .. . [6] R. J. Huffman, Y. Tanaka, and J . C. Larrabee, J. Opt . Soc. Am. 52, 851 (1962). [7] J . J. Hopfield, Phys. R ev. 35, 1133 (1930); ibid, 36, °1 ~37~5~------~1735~0~------1~3~25~------~1 3~0 0~--' 784 (1930) . WAVELENGTH 1\ [8] Y. Tanaka, J. Opt. Soc. Am. 45, 710 (1955). [9] Y. Tanaka, A. S. Jursa, and F. J. LeBlanc, J. Opt . Soc. FIG U RE 10. E nlarged-scale plot of the initial pm·tion of an Am. 48, 304 (1958). ionization efficiency curve fo r the C6 D.+ ion showing the [10] Y. Tanaka, and M . Zelikoff, Phys. Rev. 93, 933 (1954); vibrational intervals of the g1'ound stale of the ion. J. Opt. Soc. Am. 44, 254 (1954) . [11] P . C. Wilkinson and Y. Tanaka, J . Opt. Soc. Am. 45, 344 (1955). 416 [12] F. S. Johnson, K Watanabe, and R. Tousey, J. Opt. [25] A. D. M cLean, J. C hern. Phys. 32, 1595 (1960) . Soc. Am. H, 702 (1951). [26] J. E. Collin, Bull. Soc. Chim. Belg. 71, 15 (1962). [1 3] S. W. Wonssowski, A. W. Sokolow, and A. S. Wexler, [27] E. Lindholm, Proceedings of t he Eleven t h Annual Fortschr. Physik, 4, 11 5 (1956); ibid. 4, 216 (1956). Conference on Mass Spectrometry a nd Allied Topics, [14] I-I. E . I-linteregger and K. Watanabe, J. Opt. Soc. Am. San Fra ncisco, Calif. May 19-24, 1963. 43, 604 (1953) . [28] W. C. Price and R . W. Wo od, J . C hcm. Phys. 3, 439 [15] R eference [1], p . 354. (1935). [16] E. C. Y. Inn, Sp ectrochimica Acta 7, 65 (1955) . [29] P. G. 'Wilkinson, J . Chcm . Ph ys. 24, 917 (1956); Can. J . [17] K . Watana be, F . F . Marmo, and E. C. Y. Inn, P hys. Phys. 34, 596 (1956). R ev. 91, 11 55 (1953). [30] M . F . AmI' E l-Sayed, M. K asha, a nd Y. Tanaka, J . Chem . [18] A. J. C. Nicholson, J. Chem. Phys. 39, 954 (1963). Phys. 34, 334 (1961). [19] W . C. Price, Phys. Rev. 47, 444 (1935) . [31] K. Goto, Sci. of Light, 11, 116 (1962). [20] K Watanabc, J . Ch cm. Phys. 26, 542 (1957). [32] R . E. Fox and W. M . 11 ickam, J . Chern . Phys. 22, [21] T. N akayama and 1\:. ~Watanab e, J . Che rn . Phys. 40, 2059 (1954). 558 (1964). [33] J . D . Morrison, J. Chern. Phys . 22, 1219 (1954). [22] F . P . Lossing, A. W . Tickner, a nd W. A. Bryce, J . Chem. Phys. 19, 12M (195 1). [23] P . G. Wi lkinson, J. Mol. Sp ectroscopy 2, 387 (1 958). [24] R. S. Mu ll iken, Can. J . Chcm. 36, 10 (1958). (Paper 68A4- 289)

417 JO URNAL OF RESEARCH of the Na tional Bureau of Sta ndards-A. Physics and Chemistry Vol. 6SA, No. 4, July-August 1964

Publications of the National Bureau of Standards*

Selected Abstracts These cataloged references include radiation field studies ran o'i ng from the point-source infini te-medium situation up General application of Youden's rank sum test for outliers t hr; ugh such co mplicated geometries as foxholes, shelters, and tables of one-sided perce ntage points, T . A. Willke, J . a nd conventional structures. The other references are of a R es. N B S 68B (1lfath. and M ath. P hys.), No.2, (ApT.­ ge neral or review nature or contain input spectral data. J un e 1964) . 75 ccnts. The rank sum test fo r outliers advanced by ,V. J . Youden Inspection of processed photographic record film s for aging provides a method for detectin g if the measurement distri­ bl e mishes, C. B. l\IcCamy, N B S H andb. 96 (Jan. 24, 1964), bution of a nyone of a group of objects has a mean signif­ 25 cenls. ica ntly differen t from the rest. This paper disc usses a Inspections of m icro fI lms have recently revealed J:> lemishes more general a pplication of t he rank sum procedure which whi ch ap parently developed 2 to 20 years after t he [llms were permits a similar test on other para meters, such as t he processed. Most of the blemishes a re small spots, usually variance, with t he same tables. T ables of t he critical values reddish or yellowish in color, ra nging from about 15 to of the extreme rank sum a nd t he correspondi ng significance 150 microns across. These blemishes have been classified levels for one-sided tests arc given in this paper to supple­ on the basis of size, sha pe, color, a nd character. This publi­ men t similar tables for two-sided tests already publis hed. cation describes and gives colored ill ustrations of the various types, describes the method of observing t hem, and recom­ mends sampling procedures for mi crofilm inspectors. The Fitting y = {3 x wh e n the vari ance depends on x, J . Van D yke, cause, t he exact mechanism of formation of t he various types, J . Res. N B S 68B (lVIath. and lYl ath. Phys.), N o.2. (A pi·.­ a nd generally accepted preventive measures are not as yet J une 1964). 75 crnts. k nown. This p ublication is intended to promote ul1lform T his paper presents some results co ncernin g the selection of termi nology, inspection, and reporting. a method for eslimating the slope of a straight line through t he ori gin. F or filli ng the lin e y = {3x when the variance of Safe handling of radioacti ve material, NBS IIandb. 92 y is p roportional to X", it is we ll. k nown that the best estimate (M ar. 9, 1964), 40 cents (Supersedes H andb. 42). of {3 depends on p. I n practice, however, only in teger values This ha ndbook presents recommendations of the National of p would be convenient to work with. One of lhe esti­ Commi ttee on Hadiat ion Protection and Measurements. mators appropr iate for p = 0, 1,2 wou ld probably be used if It is designed to help the user of radioactive materials to t he value of p were in fact fractional, or if it were only ap­ handle radiolluelides without exposin g himself or others to proximately known. This paper p rovides some guides for doses in excess of maximum perm issible lim its. I t provides choosing t he best a mong these estimators in a particular background material on the prin ciples of radiation protection situation. a nd then

On the relaxation of the hard-sphere Rayleigh and Lorentz The use of an analog computer in side-on arc , gas, K. Anderson and K. E. Shuler, J. Chem. Phys. 40. No.3, J. B. Shumaker, Jr., and C. R. Yokley, Appl. Opt. 3, No.1, 633-650 (Feb. 1, 1964) . 83-87 (1964) . As part of a study of the relaxation of non-equilibrium sys­ An analog computer which solves Abel's integral equation is t ems, the (translational) relaxation of a hard sphere Rayleigh described and its appli cation to the side-on spectroscopic and Lorentz gas is investigated. A master equation is study of high current arcs is illustrated. By its use radially formulated to represent the time variation of the distribution resolved spectra of cylindrically symmetric inhomogeneo us function of the sUb-system particles and a technique is devel­ sources are obtained with the sam e speed and simplicity as oped for transforming this integral master equation into are spectra of homogeneous sources. Comparison of NI differential Fokker-Planck equations. The Fokker-Planck transition probability measurements using this computer equation for the Rayleigh gas is solved analytically and ex ­ with those using conventional numerical radial resolution plicit solutions are presented for the relaxation of initial techniques indicates that the computer errors are negligible. Maxwell and initial ii-function distributions of the energy. The Fokker-Planck equation for the hard sphere Lorentz The direct determination of the crystal structure of gas does not appear to be susceptible to an analytical solution NaB(OH),. 2H20, S. Block and A. P erloff, Acta Cryst. 16, in t erms of orthogonal polynomials, but some general No. 12, 1233-1238 (Dec . 1963). properties of the solution have been established. The structure of NaB(OH),.2H20 has been determined by direct methods. Three-dimensional Cu Ka single-crystal Radiation-induced at high pressures. L. A. data were used. The space group is PI, Z= 2 and the cell Wall and D. W. Brown, J . Polymer SC?:.: Pt. C, No. 4, 1151- dimensions are: 1160 (1 963). Several radiation-induced polymerization reactions were a= 6·126 ± 0·008 A a = 67 °55' ± 07' investigated at pressures between 5,000 and 17,000 atmos­ b= 8·l80 ± 0·008 A (3 = 110°35' ± 07' pheres and temperatures between 20 °C and 275°C. The c= 6·068 ± 0·008 A 1'= 101 °51' ± 07' most detailed studies have been with propylene and n-tetradecafluoroheptene-1. With both monomers the variation The structure consists of discrete tetrahedral B(OH); groups of rate of polymerization, R v , with temperature and radiation intensity is consistent with a free radical mechanism. The and octahedrally coordinated sodium ions. An octahedron maximum number of monomer units converted to polymer shares two edges to form chains parallel to the [100] direction. per 100 electron volts absorbed, G(M), is 10R,000 and 7,600 The chains are linked in the [001] direction by boron tetra­ in the polymerization of propylene and the heptene, respec­ hedra to form sheets parallel to the (010) face. Hydrogen tively; thus, the kinetic chains are long. However, t he bonds connect oxygen within the sheet and link the number-average degree of polymerization, D P n, is low, t he sheets into a three-dimensional network. Bond distances and maximum values being about 75 in both polymerization. bond angles are presented and t he application of the direct In the polymerization of propylene, transfer to monomer method is discussed. limits DPn ; the transfeT constant becomes greater as tem­ perature increases or pressure decreases. In the polymeriza­ Wave functions and oscillator strengths for the lithium iso­ tion of the heptene, DPn is determined by both transfer and electronic sequence, A. W. Weiss, A strophys. J . 138, No.4, termination. In the later polymerization both Rv and DPn 1262-1276 (Nov. 16,1963). first increase and t hen decrease as the temperatures is raised Hartree-Fock wave functions for the l s22s, l s22p, l s23s, at constant pressure because the depolymeTization rate l s23p, and l s23 d configurations of the lithium isoelectronic becomes appreciable at high temperature a nd reduces the sequence through Z = 10 have been calculated. Oscillator observed R p. strengths for all possible transitions between t hese levels were computed utilizing the dipole length, velocity, and accelera­ Pyrolysis of polytrifiuoroethylene: influence of gamma tion forms of the transition matrix element. In addition, the radiation and alkali treatment, S. Straus and L. A. Wall, 2s-2 p transition has been calculated using a 45-term con­ SPE Trans. 4, No.1, 61- 65 (Jan. 1964). .figuration interaction function for both states. Pyrolysis in vacuum of gamma-irradiated polytrifluoroethyl­ ene indicates increased rates of volatilization with increased Thermal expansion of silver iodide, A. Bienenstock and G. dosage. At the higher radiation doses, t he rate curves no Burley, J. Phys. Chem. Solids 24,1271- 1278 (Apr. 1963). longer show a maximum but resemble those previously The thermal expansion of silver iodide with the zincblende obtained with branched . structure has been measured by X-ray diffraction t echniques Studies of swelling with indicate crosslin king, which in the temperature range 4.2 to 300 OK. Although dependent increases with radiation dose. It is estimated that for each on sample preparation, it is negative to approximately 80 OK , chain fracture caused by radiation there are approximately positive in the range 80 OK to 110 OK , and then attains a seven crosslinks formed. However, in addition to the cr088- constant negative high t emperature value. This expansion links a branched structure is also being produced as evidenced is explained in terms of a model which shows the dependence by the character of the thermal degradation rate curves. of thermal expansion on structure and bonding. It is shown Alkali t reatment of t he polymer sensitizes it towards de­ that the primary role of covalency, in its contribution to the hydrofiuorination and double bond formation. Subsequent low-temperature contraction, is to r educe the interaction rate and pyrolysis studies showed t he formation of very stable between next-nearest neighbors. The particular form of the residues, indicating a relatively greater thermal stability of interatomic interaction appears to contribute primarily to the main chain. the balance between modes with positive and negative 420 Gruneisen factors. I n the case of AgI, it is the large n in still r equired to be linear in the Vi. F urthermore, Onsager's the nearest neighbor repulsive interaction, r-n , which ac­ equations are obtained while a llowing t he phenomenological counts for the negative high temperature limit. The positive matrices to be functions of the variables ai, and not merely thermal expansion at intermediate temperatures is the res ult of constants of the motion. This serves to generalize and of transverse optical modes with lower frequencies than extend Zwanzig's earlier treatment. transverse acoustic modes. Infrared spectra of HNC from 2000 to 3000 em- I, A. G. Some properties of polystyre ne networks formed from Maki and L. R. Blaine, J . Mol. Spec try. 12, No. 1, 1;5-68 oriented chains, J. C. Halpin and L. Mandelkern, J . Polymer (J an. 1, 1961;) . Sci.: Pt. B, 2,139-11; 2 (1 961;). New measurements arc reported on eight vibration-rotation The increase in isotropic length, L I , of highly oriented non­ absorption bands of H CN in the infrared region from. 2000 to crystalli ne polystyrene fibers was studied so as to compare 3600 cm- '. Comparison is made with the ipredictions of its behavior with that of previously st udicd crystalline Rank et al and ome revisions of rotational constants is found poly mers such as natural rubber and . It was necessary. P erturbations due to l-type resonance are taken shown, as predicted by t heory, that the high axial orientation into account and found to be in very good agreement with t he imposed on the system prior to network formation, and not theory. The splitting of the!:;. energy levels is fou nd to be the crystallization in the polymer, is the prime requirement large enough to be observed at microwave frequencies. to produce a substantial increase in L I . Subsequent to t he initial shrinkage, reversible and reproducible length-tempera­ Structure beyond the ionization limit in inelas tic electron ture measurements could be made. The glass temperature, scattering in the rare gases, C. E. Kuyatt and J . A. Simpson, T ., increases linearly with the degree of crosslinking. Proc . 6th I ntem. Symp. Ionization Phenomena in Gases I All, 33-35 (Paris, 1961;). Pair correlations in closed-shell systems, M. Krauss and A. The intensity of inelastic scattering of electrons with 500 to W. Weiss, J. Chem. Phys. 40, No.1, 80-85 (Jan. 1, 1961;). 1000 e V primary energy by rare gases has been examined in P air function eq uations have been derived exactly for a an electron spectrometer with a resolution of ",0.7eV. At restricted class of tri al fun ctions containing only elosed shell energies beyond the first ionization limit, structures localized pair correlations. In a matrix representation, it is shown in energy are detect ed. The structures in argon, neon, that they reduce to a homogeneous pseudo-eigen valu e krypton, and xenon occur in a region a few e V below the LJ, equation. T he results are eq ually applicable to any set of M" N" and 0 1 ionization edges respectively, and probably unitarily t ransformed set of Hartree-Fock orbitals. Com­ correspond to di screte a utoioni zing states of the inner electron parisons are made with t he Sinanoglu scheme, and compu­ involved. The structure in helium has been discussed tational problems are discussed. recently by Fano, and arises from interference between a two-electron autoioni zin g state and a continuum. Because energy losses corresponding to extreme ultraviolet transitions Microwave spectrum of lithium chloride, D. R . Lide, Jr., P. arc easily accessible, electron scattering provides a versatil e Cahill, and L. P . Gold, J . Chem. Phys. 40, No. 1, 156- 159 method for t he study of effects far out in the continuum. (J an, 1, 1961;). The rotational transit ions J = 0-> I in t he three lowest 7 Absolute oscillator s trengths for Fe I , C. H. Corliss and vibrational states of Li Cl35 ancl t ile two lowest vibrational B. Warner, Ast1'Ophys. J . Suppl. Series No. 83, VIII, 395-1;38 states of Lj7C[37 have been measured at a temperature of (Feb. 1961;). 600°-800 ° C. The molecular co nstants (in Mc/sec) for Relative gf-values for 2000 li nes of F e r between 3100 and 9900 Lj7C[35 are: B,= 21181.1 ± 0. 1, a ,= 240.2 ± 0.2, -y,= 1.2 ± 0.2 ; those for Li7C[37 are: B ,= 20989.9 ± 0.1, a ,= 236.9 ± 0.2. A from several in vestigations have been reduced to t he absolute scale of t he National Bmeau of Standards tables of The internuclear d istance is 1',= 2.02067 ± 0.00006 rt (The tra nsition probabilities. They include new observations of u ncertainties represent maximum limits of error.) A co m­ fa int li nes in the visible and infrarcd portions of the spc.ctrum. bination of the present result with molecular beam electri c resonance data yields a d ipole moment 1-' .= 7.075 + 0.0885 (V+7~) D for Li 6C135. Two high-temperature microwave Other NBS Publications spectrometers of new design are described briefly. A tabula­ t ion of molec ular constants for the complete ser ies of a lk a li J. Res. NBS 68B (Math. and Math. Phys.), No.2 (Apr.­ halide molecules is included. June 1964), 75 cents. Determinations based on dupli cation of readings. J . A. Preparation and properties of difluoroborane, T. D . Coyle, Speckman. J. J . R itter, and T . C. Farrar, Proc. Chem. Soc. (London), p. 25 General appli cation of You den' rank sum test for outliers (Jan. 1961;). and tables of one-sided percentage points. T . A. Willke. Difluoroborane (HBF2), the first exa mple of a partially (Sec above abstracts.) fluorinated boron hydride, has been prepared by t wo routes. A ge nera li zation of Rennie's inequali ty. A. J . Goldman. Some physical a nd chemi cal properties characteri'l, in g t hi s On t he asymptotic joint normality of quantil cs from a compound are given. multivari ate d istribution. L. Weis . F ittin g y = fJx when t he variance depcnds on x. J . Van Dyke. Antireciprocity and memory in the statis tical approach to (See above abstracts.) irreversible thermodynamics, R. E. Nettleton, J . Chem. Existence of k-edge connect ed ordin ary graphs with prescribed Phys. 40, No. 1, 112-116 (J an. 1961;). degrees. J . Edmonds. The macroscopi c state of a large, closed system is specified Inequalities for solutions of mixed boundary value problems b y the ensemble a CJ'ages

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