Photoelectron spectra and partial photoionization cross sections for NO, N2O, CO, CO2 and NH3

1. Quonr. Spectrosc. Radiar. Transfir. Vol. 12, pp. 59-73. Pergamon Press 1972. Printed in Great Britain

PHOTOELECTRON SPECTRA AND PARTIAL PHOTOIONIZATION CROSS SECTIONS FOR NO, N,O, CO, CO, AND NH3* J. L. BAHR, A. J. BLAKE,J. H. CARVER,J. L. GARDNER?and VIJAYKUMAR Department of Physics, University of Adelaide, Australia (Receiued 2 November 1970) Abatraet-Photoelectron spectra have been obtained for NO, N,O, CO, CO2 and NH, over a wide range of photon wavelengths using a dispersed light source. The observations cover the wavelength range 584 to 890 A and photoelectron spectra have, in general, been recorded at 5 A intervals in order to survey broad variations in the different photodisintegration processes. Partial photoionixation cross sections have been measured for all these gases for processes involving transitions to the ground states and the various excited electronic states of the residual ions; in the case of ammonia, cross sections for dissociative photoionixation have also been determined.

INTRODUCTION PHOTOELECTRONspectroscopy

provides a powerful method for studying the photoionization of atoms and molecules. This technique can be used to determine partial photoionization cross sections corresponding to transitions to particular quantum states of the residual ions and it is also possible to study processes such as dissociative ionization when they produce identifiable groups in the photoelectron energy spectrum. If a vacuum monochromator is used as the light source the variation with photon energy of the various partial cross sections can be investigated over a wide range of photon wavelengths limited only by the lamp intensity spectrum and the sensitivity of the system used to energy analyse and to detect the photoelectrons. We have previously described the use of a vacuum monochromator, together with cylindrical’ ’ ) and hemispherical”’ retarding potential photoelectron spectrometers to determine partial photoionization cross sections for molecular oxygen, nitrogen, and water vapour (‘) and for carbon dioxide.t2) In the present paper we continue these studies of the photoionization of small molecules which are of importance in studies of planetary atmospheres. The object of this investigation was to determine broad trends in the various electronic transition probabilities over a wide range of photon wavelengths. Since detailed knowledge of the vibrational structure is not essential for this study, the energy resolution has been reduced in order to improve the electron count rate. We have used higher resolution

* This work has been supported by the Australian Research Grants Committee and by the U.S. Air Force Office of Scientific Research under Grant No. AFOSR-68-1568. t CSIRO Postgraduate Student. 59

60

J. L. BAHR, A. J. BLAKE, J. H. CARVER,J. L. GARDNER and WAY

KUMAR

to investigate the vibrational structure in detail at a restricted number of wavelengths near autoionized resonances and we will report those results separately. This paper describes the determination of partial photoionization cross sections for nitric oxide, nitrous oxide, ammonia and carbon monoxide. Additional photoelectron spectra for carbon dioxide are also presented and the previously reported”’ results for this gas have been revised to take more precise account of the electron collection efficiency of the hemispherical analyser. The present results cover a range of photon wavelengths from 584 to 890 A. EXPERIMENTAL

METHOD

The experimental arrangements were essentially the same as those described previously.(‘*2) The light source was the Hopfield continuum of helium, dispersed by a 1 m vacuum monochromator (McPherson Model 225) the exit slit widths of which were set to a wavelength resolution of 8 A. A staircase retarding potential was applied to the spherical analysing grid of the photoelectron spectrometer and, as described previously’2’ repeated scans of the photoelectron spectrum were made at a fixed photon wavelength. The potential of the analysing grid of the photoelectron spectrometer was further modulated by a square wave voltage which was used in conjunction with the add-subtract modes of a multichannel analyser (RIDL 34-12B) to record differential photoelectron spectra at finite energy intervals. The spectra were each recorded in 100 channels of the multichannel analyser. Thus if a staircase potential swing of 10 V was necessary to stop the electrons of highest energy, adjacent points in the spectra were separated by 100 mV. The electron detector was a Mullard channel electron multiplier (Type B419BL). The relative collection efficiency of the spectrometer for photoelectrons of different energy was determined by recording the photoelectron spectrum of argon at a series of photon wavelengths. The relative light intensity was measured at each wavelength and the relative collection efficiency of the spectrometer was determined by comparing the measured photoelectron collection rates with the photoionization cross section measurements of COOK et aLC3)The collection efficiency of the photoelectron spectrometer was found to be effectively constant for the higher energy photoelectrons (about 5 V). The collection efficiency increased slowly with decreasing photoelectron energy in such a way that 0.5 V electrons were collected about twice as efficiently as were 5 V electrons. For electrons of very low energy (less than 0.1 V) the collection efficiency increased rapidly, possibly as a result of residual contact potentials in the spectrometer. The present measurements have been corrected for variations in electron collection efficiency and the results have been analysed and plotted using the CDC 6400 computer of the University of Adelaide. EXPERIMENTAL

RESULTS

AND

ANALYSIS

Nitric oxide NO Photoelectron spectra for nitric oxide were recorded at the 584A helium resonance line and at 5 A intervals in the wavelength range from 605 to 790 A using the dispersed Hopfield continuum. At longer wavelengths the X3x+ ground state of NO+ is the only

Photoelectron spectra and partial photoionization

cross sections for NO, NzO, CO, COz and NH,

61

electronic state which is energetically accessible. Photodisintegration reactions which may occur for nitric oxide within the wavelength range of these experiments are listed in Table 1. It is apparent that with the fairly broad energy resolution (-0.2 eV) of the present survey experiment not all the electron states of the NO+ ion can be resolved and in analysing the results of the measurements we have grouped together several states which could not be separated completely.

TABLE 1. PHOTODISINTEGRATM)NPROCESSESIN

NO

Ionization potential (eV) Process NO(X’I-I)+hv+ NO+(X’Z+)+eNO+(&+)+eNO+(bTI)+eNO+(w3A)+eNO+@‘%-)+eNO+(A”Z-) + eNO+(W’A)+eNO+(A’fI)+eNO+(c3fI)+e-

EDQVIST et al.‘4b

9.267 15.649 16.558 16%0 17.585 17.820 (18.07) 18.322 20.41

Present work (estimated error * 0.1 eV) 9.3 15.7 16.6

18.3

The measured photoelectron spectra are shown in Fig 1. The spectra in each section of the figure are normalized to the same area so that, as the wavelength varies, the changing distribution among the electronic states can be clearly seen. The plots were obtained directly from the CDC 6400 computer. The X3C+ ground state of the NO+ ion was clearly resolved at all the wavelengths which were investigated. At wavelengths shorter than 610 A transitions to the c311 state of NO+ produce a clearly resolved photoelectron peak. Transitions to the (a3E+ +b311) states of NO+ could not be completely separated individually and were analysed as Group A. For a similar reason transitions to the (w3A+W3Z- + A”Z- + W’A + A’II) states were analysed as Group B. The branching ratios for transitions to the X3x+ state, the c311 state, and Group A and Group B of the unresolved states are shown in Fig 2 The partial photoionization cross sections for nitric oxide shown in Fig. 3 were obtained by multiplying the measured branching ratios by the total photoionization cross sections of &OK et ~1.‘~’ averaged over 8 A wavelength intervals to correspond with the resolution of the present experiment. The strong peak in the total photoionization cross section near 780 A is observed in the partial cross section for the X3x+ ground state and, to a lesser extent, in that for the a3Z+ state included in Group A. The X3Z’ partial cross section declines slowly towards shorter wavelengths throughout the range studied and this reflects the increasing competition from transitions to higher states as they become allowed energetically. The partial cross section for the combination of states included in Group B varies between 5 and 6 M b from 584 A to the threshold of the A’II state at 677 A. At longer wavelengths where fewer states can contribute the Group B cross section decreases slowly towards zero at 735 A.

J. L. BAHR, A. J. BLAKE,J. H. CARVER, L.J. GARDNER and WAY KUMAR

62

NITRIC

OXIDE

/\

A

C7”

o:o ENERGY

Lo

$0

(VOLTS)

zzYT--&--”

G---70-!‘!

00

4.0 ENERGY

CV&ooS

ENERGY

120

(VOLTS

1

)

FIG. 1. Photoelectron

spectra for nitric oxide for wavelengths in the range 584 to 780 A. The curves in each section of the figure are normalized to the same area.

NO

60

Wavelength,

fi

FIG.2. Branching ratios for the photo-production of NO’ in the X3 Z+ and c”fJ states, and the groups of unresolved states A and B. The electronic states contributing to these groups are in-

dicated in the text.

Photoelectron spectra and partial photoionization

cross sections for NO, N,O, CO, CO2 and NH,

x

63

x31+

o Group A 0

12

Wavelength,

Group B

8

FIG. 3. Partial photoionization cross-sections for the production of NO+ in the X’Z+ and c311 states and the groups of unresolved states A and B. The upper curve is the total photoionization cross section of NO obtained by averaging the results of Coon et t~/.‘~)over 8 A intervals.

Nitrous oxide N,O Photoelectron spectra for nitrous oxide were recorded at 584 A and at 5 A intervals throughout the wavelength range from 600 to 765 A; at longer wavelengths the only transitions possible are those to the X’II ground state of the N20+ ion. The reactions listed in Table 2 were observed at all wavelengths where they were energetically allowed. The observed photoelectron spectra for nitrous oxide are shown in Fig. 4 and the derived branching ratios for the various electronic transitions are shown in Fig. 5. The branching ratio for transition to the ‘ZJ state of N20+ rises sharply from its threshold at 765 A and between 710 and 750 A transitions of the ‘C: state and the X2fI ground state of N,O+ are almost exactly equally likely. The branching ratio for transitions to the B state of N20f also rise sharply from its threshold at 620 A, the increase being mainly at the expense of the “C: state. The sharp rise from threshold seen for the ‘Ez and B state transitions contrasts strongly with the behaviour for the A state. The branching ratio for transitions to the A state of N20+ increases very slowly from the threshold at 710A and the nature of the electron energy TABLE2. PHOTODISIN~EGRATION PROCESSES

IN

N20

Threshold energy (eV) Reaction NzO+hv +N20+(XzIT)+e-rNzO+(‘Z:)+e+N,O+(A)+e-

-+N,O+(B)+e-

Present work BRUNDLEand TURNER(~)(estimated error +O.l eV) 12.89 16-38 17.5 (18.23 vertical) 20-11

12.9 16.4 (18.2 vertical) 20.1

64

J. L. BAHR, A. J. BLAKE,J. H. CARVER,J. L. GARDNERand VIJAY KUMAR

NITROUS

0.0

zo 40 ENERGY (&I

80

0x10~

lG0

FIG. 4. Photoelectron spectra for nitrous oxide for wavelengths in the range 584 to 795 A. The curves in each section of the figure are normalized to the same area.

N2O

Wavelength,

FIG. 5. Branching ratios for the photo-production

%

of N20f

in the X’lT, ‘Z:, A

and B states.

Photoelectron

spectra and partial photoionization

cross sections for NO, N,O, CO, CO2 and NH3

65

spectrum in the threshold region is very interesting The onset and development of the broad photoelectron energy peak corresponding to the formation of NzO+ in the A state is clearly seen in Fig 4 at photon wavelengths shorter than 710 A. Inspection of the photoelectron spectra at short wavelengths (below 66OA) where the photoelectron peak, corresponding to transitions to the A state, is fully developed shows that this peak has a full width at half-maximum of about l.OeV. Thus there is a large difference between the ‘vertical’ and ‘adiabatic’ ionization potentials for this transition. As the photon wavelength decreases below the adiabatic threshold the maximum in the photoelectron energy spectrum remains near zero energy until a photon energy equal to the vertical ionization potential is reached. This is the explanation for the maximum near zero energy in the photoelectron spectra taken between 680 and 710A (Fig 4). Below 680A the maximum in the A peak moves to higher photoelectron energies with decreasing photon wavelength in the normal way. Cross sections for the various competing photoionization processes in N,O are shown as a function of wavelength in Fig 6. These partial cross sections were obtained by multiplying the branching ratios of Fig. 5 by the total cross sections of Coon et ~1.‘~)averaged over 8 A intervals to correspond with the wavelength resolution of the present measurements. The total cross section curve at this broad resolution shows considerable variations which are reflected in the partial cross sections in the wavelength range between 600 A and the threshold for the first excited state of N,O+ at 765 A.

Wavelength.

a

FIG. 6. Partial photoionization cross sections for the production of N20+ in the X’II, ‘Z:, A and B states. The upper curve is the total photoionization cross section of N20 obtained by averaging the results of Coon et u!.(~) over 8 A intervals.

66

J. L. BAHR, A. J. BLAKE, J. H. CARVER, J. L. GARDNER and WAY

KUMAR

Carbon momxcide CO

Photoelectron spectra for carbon monoxide were recorded at 584 A and at 5 A intervals between 6OOA and the threshold for the first excited state of CO+ at 755 A. The spectra are shown in Fig 7. At all wavelengths where they were energetically allowed peaks were observed corresponding to the photodisintegration processes listed in Table 3. In the wavelength region between 645 and 670 A (Fig. 7) the photoelectron peak corresponding to transitions to the A’II state of CO+ is broadened considerably compared with its shape at longer wavelengths. This broadening can be attributed to enhancement of higher vibrational levels of the A state due to indirect transitions via an autoionizing state. We have also observed similar enhancement of the X2x+ state at wavelengths between 790 and 85OA where transitions to the X2x+ state are the only ones possible energetically. TABLE 3. PHOTODISINTEGRATION1~ CARBON MONOXIDE

Threshold

Reaction

TURNER(~)

COLLIN”’

energy (eV)

Spectroscopic(*’

Present work (estimated error +O.l eV)

14.013 16.536 19.674

14.0 16.5 19.7

CO(X’~.+)+hv

+CO+(X2Z+)+e+CO+(A2rI)+e+CO’(B*Z’) +e-

CARBON

14.00 16.54 19.65

MONOXIDE

ENERGY FIG. 7. Photoelectron

14.01 16.55 19.69

745 0

(VOLTS I

spectra for carbon monoxide for wavelengths in the range 584 to 745 A. The curves in each section of the figure are normalized to the same area.

Photoelectron spectra and partial photoionization cross sections for NO, N,O, CO, CO, and NH3

67

and NATALIS”) have reported an anomalous intensity distribution among the vibrational levels of the X state CO+ in spectra obtained with an undispersed neon discharge in the wavelength range 736744 A. The spectra obtained near these wavelengths show structure in the photoelectron intensity in the energy region between the maxima of the X’Z’ and A2fI peaks. Following the observations of COLLIN and NATAL&‘) these electrons have been included in the branching ratio of the X2x+ state. Branching ratios for transitions to the X2x+, A2TI and B2Ef states of CO+ are shown in Fig. 8 as a function of photon wavelength. Some earlier results obtained at a few wavelengths by SCHOEN(‘O) using a retarding potential analyser of cylindrical geometry are shown for comparison in Fig 8. There is good overall agreement between the two sets of measurements if allowance is made for the greater detail ofthe present results. The branching COLLIN

Wavelength,

FIG.8. Branching

ratios

I!

for the photo-production of CO+ in the X’Z+, A’fl and B’Z+ states. The dashed lines are the results of SCHOEN.“~)

ratios measured in the present experiment have been combined with the total photoionization cross sections determined by COOK et aLt3’ (averaged over 8 A intervals) to obtain the partial photoionization cross sections shown in Fig 9. Figures 8 and 9 show that the intensity of the A’II transitions increase slowly from the threshold at 755 A. This effect is the result of the width of the A’II photoelectron peak in which the transition probability is distributed broadly over a large number of vibrational levels which are unresolved in the present experiment. Thus as the photon wavelength decreases from 755 A more vibrational states become available for ionization and the relative cross section for the whole electronic state increases ideally in a step-like manner.

68

J. L. BAHR, A. J. BLAKE, J. H. CARVER,J. L. GARDNERand VIIAYKUMAR

oT

Wavelength,

A

FIG.9. Partial photoionization cross-sections for the production of CO+ in the X’Z’, A’fl and E’Z+ states. The upper curve is the total photoionization of CO obtained by averaging the results of Coon et t~J.‘~i over 8 A intervals.

The partial cross sections for the X2x’ and A21T states are approximately equal at wavelengths shorter than 720 8. The B’IZ+ cross section increases slowly from its threshold at 635 a and never amounts to more than 10 per cent of the total cross section at the wavelengths investigated in this work. Carbon dioxide CO,

Photoelectron spectra for carbon dioxide were recorded at 584 A and at 5 A intervals between 610 and 715 A; at longer wavelengths only the 21Tgground state of CO: is energetically accessible. The reactions listed in Table 4 were observed at all wavelengths where they were energetically possible. TABLE4. PHOTODISINTEGRATION PROCESSES

IN CARBON

DIOXIDE

Energy threshold (eV) Reaction CO,(Ground State) + hv -+CO~(211,)+e+CO:(2Tl,)+e-+cof(%:)+e-CO:(2Z:)+e-

TANAKA AL-JOBOURYTURNER et al. (i2) et aI.“3’ et al.” ‘I 13.78 * 1805 19.39

13.68 17.27 18.08 19.29

* MROZOWSKI(‘~) predicted this level to be at 17.32 eV.

13.79 17.32 18.07 19.36

ELAND et a1.“4’ 13.78 17.32 18.05 19.36

COLLIN et al.@’ 13.84 17.8 18.1 19.4

Present work 13.7 17.3 18.0 19.3

Photoelectron spectra and partial photoionization

cross sections for NO, N20, CO, CO2 and NH3

69

The observed photoelectron spectra for carbon dioxide are shown in Fig. 10. These results have been corrected, as explained above, for variations in the electron collection efficiency of the spectrometer but the general form of the spectra and the broad conclusions about the competition between different transitions are the same as described previously.“) The onset of ionization to the 2YLc, state of CO: at wavelengths shorter than 610 A is shown in Fig. IO. The rising importance of the 211Ustate at the expense of the X211, state is clearly seen in Fig. 10 and also in the branching ratios shown in Fig. 11. These branching ratios have been combined with the total cross sections determined by COOK et ~1.‘~)(averaged over 8 A) to obtain the partial cross sections shown in Fig. 12. It can be seen that at wavelengths shorter than 680 A the most likely photoionization process involves transitions to the ‘Xi excited state of CO:. CARBON

DIOXIDE

0 ENERGY FIG.

t VOLTS 1

ENERGY

1 VOLTS 1

10. Photoelectron spectra for carbon dioxide for wavelengths in the range 584 to 720A. The curves in each section of the figure are normalized to the same area.

Wavelength. FIG. 11. Branching ratios for the photo-production

a

of CO: in the ‘IIs, *II”, ‘Z: and ‘Zl states.

J. L. BAHR, A. J. BLAKE, J. H. CARVER, J. L. GARDNER and VLJAYv*rs-rn

70

Wavelength.

%

FIG. 12. Partial photoionization cross sections for the production of CO: in the 2fIa, ‘fl., ‘IZ: and “Xl states. The upper curve is the total photoionization cross section of CO2 obtained by arranging the results of Coon et .1.‘3’ over 8 A intervals.

Ammonia NH, Photoelectron spectra for ammonia were recorded at 584 A and at 5 A intervals in the range 625-890 A. The spectra are illustrated in Fig. 13 and they show several well-defined peaks. The most energetic group of photoelectrons is identified with transitions to the ground AMMONIA

ENERGY IVOLTS)

ENERGY

IVOLTS)

ENERGY

~VOLTSI

ENERGY

FIG. 13. Photoelectron spectra for ammonia for wavelengths in the range 584 to 895 in each section of the figure are normalized to the same area.

(VOLTS

A.The

I

curves

Photoelectron spectra and partial photoionization cross sections for NO, N,O, CO, CO2 and NH,

71

state of NH:. The position of the maximum of this group corresponds to a vertical ionization potential of (10.6 f O-1)eV and the width of the peak suggests an adiabatic ionization potential of (10-l +0-l) eV for the ground state of NH: which is in good agreement with the value of 10.16 eV found by DIBELER et a1.‘r6’and consistent with F’RICE’S(’‘) measurements. WATANABE~“) found a difference of O-5eV between the adiabatic and vertical ionization potentials in agreement with the present results. At 895 A a new peak appears which remains close to zero energy until 840 A (Fig. 13), when it begins to move toward higher energy in the normal manner. At shorter wavelengths the peak has an energy which indicates an adiabatic ionization potential of (14.8 + 0.15) eV. too

-

80

\o 0

60 .P

5 .-P x

e

40

F m

20

0 600

700

900 Wavelength,

900

a

FIG. 14. Branching ratios for the photo-production of NH: in the ground state and first excited state, NH: and NH+ in their ground states and for the aoomolous process described in the text.

This is in good agreement with COOK and SAMSON’S(‘~) value of 14.9 eV for the first excited state of NH:, and we associate the peak in spectra recorded at wavelengths shorter than 840 A with this state. However, the peak which appears at low energy in the spectra recorded in the region 895-840 A must result from another mechanism, possibly the fluorescent autoionization process described by BLAKE and CARVER. (l) The total fluorescence yield for ammonia observed by COOK and METZGERU) shows a strong maximum in this region. The photoelectron spectrum obtained by PRICE(I’) at 584 A shows a broad group with a width of about 3 eV and an adiabatic ionization potential of 14.7 eV. The present results (Fig. 13) indicate that the broad group has two distinct maxima and the positions of the two peaks correspond to vertical ionization potentials of (15.6+0-l) and (16.8 f0.1) eV. The peak at 15.6 eV is attributed to transitions to the first excited state of NH: while the

72

J. L. BAHR, A. J. BLAKE,J. H.

CARVER,

J. L.

GARDNER

and WAY KUMAR

peak at 16.8 eV probably corresponds to the dissociative ionization reaction, NH,+hv

-+ NH: +H+e-.

It is difficult to distinguish clearly the onset of this peak as the photon wavelength decreases but observations near threshold together with the width of the peak at shorter wavelengths suggest an adiabatic ionization potential which is less than 16.3 eV. A fourth peak appears in the photoelectron spectra obtained at wavelengths shorter than 695 A (17.8 eV). The position of this low energy peak remains almost constant until the photon wavelengthreaches 645 A after which the peak energy increases in the normal way with decreasing photon wavelength. This peak probably corresponds to the reaction NH,+hv

+ NH++H,+e-

for which MANN et ~1.‘~~)estimated a threshold of (19.4 +_0.5) eV, a somewhat higher value than is indicated by the present work. The behaviour of the photoelectron peak in the threshold wavelength region between 695 and 645 A, where the peak position does not shift significantly with photon energy probably results from the difference between the vertical and adiabatic ionization potentials for the reaction. The present results suggest a vertical ionization potential of (18.7 +O.l) eV and an adiabatic ionization potential of (17.8fO.l) eV.

16-

x NH: l

(g.s.)

Anom.

o NH:(es) 4 NH; 0 NH+

‘6CX

h

i 700 Wavelength.

800 H

Kl

FIG. 15. Partial photoionization cross sections for the production of NH; in the ground state and first excited state, NH: and NH+ in their ground states, and for the anomolous process described in the text. The upper curve is the total photoionization cross section of NH, obtained by arranging the results of COOKet ~1.‘~’over 8 A intervals.

Photoelectron spectra and partial photoionization cross sections for NO, N,O, CO, CO1 and NH3

73

Table 5 summarizes the information obtained from the present measurements about the ionization potentials for the various processes observed in the photoionization of ammonia The branching ratios for the processes listed in Table 5 and for the anomalous process above 840 A are presented in Fig. 15. These results have been combined with the total cross section measurements of CooK et ~21.‘~’ (averaged over 8 A intervals) to obtain the partial photoionization cross sections shown in Fig. 16. Figures 15 and 16 illustrate the competition between the various photodisintegration processes in ammonia. TABLE 5. PHOTODISINTEGRATION PROCESSES IN AMMONIA

Process NH,+hv +NHf(g.s.)+e-+NH:(e.s.)+e+NHz +H+e+NH++H2+e-

Adiabatic ionization potential

Vertical ionization potential

(14.8&0+15)eV < 16.3 17.8kO.l

(10.6fO.l)eV 15.6kO.l 16.8+01 18.7kO.l

REFERENCES 1. A. J. BLAKEand J. H. CAR-, J. &em. Phys. 47, 1038 (1967). 2. J. L. BAHR, A. J. BLAKE,J. H. CARVERand VUAY KUMAR,JQSRT 9,1359 (1969). 3. G. R. COOK, P. H. METZGER,M. OGAWA, R. A. Becum and B. K. CHING, Report No. TDR-469(9260-01)-4, Aerospace Corp., El. Segundo, California (1965). 4. 0. EDQVIST,E. LINDHOLM, L. E. SELIN.H. SJORGREN and L. ASBRINK. To be published in A&iv. Fysik. 5. C. R. BRUNDLEand D. W. TURNER,J. Mass. spectrosc. Ion Phys. 2,195 (1969). 6. D. W. TURNBRand D. P. MAY, J. them. Phys. 45,471 (1966). 7. J. E. COLLIN and P. NATALIS,J. Muss specrrosc. Ion Phys. 2,231 (1969). 8. Y. YANAKA, Sci.Pap. Inst. Phys. Chem. Res. Tokyo 39,465 (1942). T. TAKAMINE,Y. TANAKAand M. IWATA, Sci. Pap. Inst. Phys. Chem. Res. Tokyo 40, 371 (1943). E. LINDHOLM,Ark. Fys. 8,433 (1954). 9. J. E. COLLIN and P. NATALIE,J. Muss Specrrosc. Ion Phys. 1, 121 (1968). 10. R. I. SCHOEN, J. them. Phys. 40, 1830 (1964). 11. Y. TANAKA,A. S. JURSAand F. L. LE BLANE,J. them. Phys. 32, 1199, (1960). 12. M. I. AL-JOEOURY,D. P. MAY and D. W. TURNFIR, J. Chem. Sot. 6350 (1968). 13. D. W. TURNERand D. P. MAY, J. them. Phys. 46, 1156 (1967). 14. J. H. D. ELANDand C. J. DANBY, J. Mass Spectrosc. Ion Phys. 1, lll(1968). 15. S. MROZOWSKI,Reu. mod. Phys. 14, 216 (1942); Phys. Rev. 60, 730 (1941): 62, 270 (1942); 72, 682 (1947). 16. V. H. DIBELJZR, J. A. WALKBRand H. M. ROSENSTOCK, J. Res. Natn. Bar. Stand. A70,459 (1966). 17. W. C. PRICE,Mofec. Spectrosc. 4,22 1 (1968). 18. K. WATANABE,J. them. Phys. 22, 1564 (1954). 19. G. R. CQOK and J. A. R. S-N, Bull. Am. Phys. Sot. 4,454 (1959). 20. M. M. MANN, A. ILL~~TRALIDand J. T. TATE, Phys. Rev. !%, 340 (1940).