Characterization of neutral species produced by electron interactions with N2, O2, CO and Ar

and Ion Processes, 56 (1984) 93-107 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands

International Journal of Mass Spectrometty

CHARACTERIZATION OF NEUTRAL SPECIES PRODUCED ELECTRON INTERACTIONS WITH N,, 0,, CO AND Ar

GERALD

D. FLESCH and HARRY

Ames Laboratory-USDOE Ames, IA 50011 (U.S.A.)

93

BY

J. SVEC

* and Department

of Chemistry,

Iowa State University,

(First received 19 July 1983; in final form 27 September I983)

ABSTRACT A neutral fragment mass spectrometer has been used to generate and characterize the neutral products formed when energetic electrons interact with N,, O,, CO and Ar and mixtures thereof. Atomic nitrogen in its ground state is produced from N,. All other neutral products observed are in excited states, primarily in atomic Rydberg states.

INTRODUCTION

In a recent publication [l] we reported that, in addition to ground-state species are formed in low abundance neutral products, “autoionizing” during the electron bombardment of M(CH,),, M = C, Si, Ge, Sn, Pb. No effort was made to characterize these species or the reactions by which they were formed. In an attempt to obtain a better understanding of the processes involved, we have sought similar reactions which may occur during the electron bombardment of simpler species such as N,, O,, CO and Ar. In this report, the characterization of several products that have been observed is described. Ground-state neutral fragments have been produced and characterized previously in dual-chambered electron bombardment ion sources [2-71. Species produced in excited states during electron bombardment of atmospheric gases have also been examined. Interest has been in identifying the metastable states generated, i.e. those states at excitation energies appreciably below the ionization energy (IP). These metastable states and energetics * Operated for the U.S. Department of Energy by Iowa State University under contract No. W-7405-Eng-82. This research was supported by the Director. for Energy Research, Office of Basic Energy Science. 01681176/84/$03.00

0 1984 Elsevier Science Publishers B.V.

94

of their formation have been studied using Auger-electron detectors [C-14], Penning ionization [Ml, and energy-loss spectra [l&18]. Long-lived, excited atomic states whose excitation energies are close to the ionization energy, i.e. Rydberg states, have also been studied [19-261. Recently, the subject has been reviewed [27,28]. Long-lived molecular Rydberg states have also been observed experimentally [29,30]. We have observed ground-state neutral fragments, metastable excited molecules, and atoms in Rydberg states (Rydberg atoms) in the dual-chambered ion source of the neutral fragment mass spectrometer. This report concerns characterization of these products with respect to appearance energies, pressure dependences, and electron current dependences. EXPERIMENTAL

The basic instrument used in these experiments has been described [1,6]. A schematic drawing of the modified instrument and the ion source is shown in Fig. 1. Sample gas collimated by a capillary array passes through the primary ionization chamber. Neutral and ionic species are produced by a pulsed electron beam (24 Hz, 5-100 eV, 20 PA average current). The pulses of positive ions are extracted from the primary chamber, mass analyzed by the primary quadrupole filter, and the ion currents recorded to assist in characterizing the reactions occurring. A fraction of the neutral species produced diffuse in pulses into the secondary ionization chamber, but charged species are prevented from doing so by potential barriers established between the chambers by the repeller electrodes and the biasing of the two ionization chambers. Ions formed from neutral species in the secondary chamber are extracted, mass analyzed, and collected. PRIMARY

4

3

SECONDARY

21

5

67

8

9

Fig. 1. Dual ion source neutral fragment mass spectrometer. 1, Primary ion source chambers ( + 5 V) with pulsed electron beam; 2, primary chamber ion draw-out (0 V) and focus ( + 2 V) electrodes; 3, primary chamber quadrupole filter (0 V); 4, primary chamber off-axis electron multiplier; 5, primary chamber repeller grid (- 45 V) and secondary chamber repeller grid (+20 V); 6, secondary ion source chamber (+20 V) with continuous electron beam; 7, secondary chamber ion draw-out (+ 15 V) and focus (+ 17 V) electrodes; 8, secondary chamber quadrupole filter (0 V); 9, secondary chamber off-axis electron multiplier.

95

The secondary ionization chamber may be operated with its electron beam “off” (Rydberg or R-mode) or “on” (normal or N-mode). In the R-mode, ions formed from atoms in Rydberg states are observed. These ions result when Rydberg atoms produced in pulses in the primary chamber approach near a metal surface [20], in the secondary chamber. When the gas pressure is sufficiently high, ions may also be produced by collisions of excited species with other species in the secondary chamber. These pulses of ions are extracted, mass analyzed (Extranuclear, Inc. quadrupole filter and power supply), and detected (off-axis Channeltron 4770). The resulting a-c. signal is amplified (laboratory-built narrow band RMS amplifier) and displayed (Oscilloscope or Houston Instrument X-Y or strip-chart recorder). When operated in the R-mode (no electron beam in the secondary chamber) the secondary chamber is the site where energetic neutral products are converted to ions by several processes. A plot of neutral product abundance vs. the energy of the primary chamber electron beam yields a neutral product efficiency (NPE) curve. The appearance energy of the reaction producing the neutral product(s) can be determined from the energy intercept of the NPE curve. In the N-mode of operation (d.c. electron beam in the secondary chamber), the neutral species arriving in pulses from the primary chamber, as well as the ambient gas molecules straying continuously into the secondary chamber, are ionized by the d.c. electron beam (2-100 eV, 10 PA). This produces, at any pertinent m/z, a combined a.c. signal (neutral species generated in the primary chamber) and d.c. signal (ambient gas). The a.c. component is extracted from the combined signal by the narrow band amplifier and displayed as the measure of the abundance of the neutral species. Ions observed while operating in the N-mode also include any ions produced from Rydberg atoms or as products of collisions of excited species. When necessary, the data due purely to neutral species produced in the primary chamber and having complementary positive ions are adjusted to correct for such contributions . When operated in the N-mode, the energy of the secondary chamber electron beam can be fixed (the secondary chamber is then used only as a detector of neutral fragments) and the energy of the primary chamber electron beam varied to yield NPE data. Conversely, the energy of the primary chamber electron beam can be fixed (the primary chamber is then used as a constant source of neutral products) and the energy of the secondary chamber electron beam varied to yield ionization efficiency (IE) data. Thus, it is possible to characterize the appearance energy of the reaction (NPE plots) and the ionization energy of the reaction product(s) (IE plots). Threshold energies are determined from NPE and IE data by an extrapo-

96

lated voltage difference method [31] when the ion abundance near threshold is sufficient to warrant its use. Otherwise the less accurate linear extrapolation method is used. The electron energy scales are calibrated through evaluation of the IE data for the molecular ions in the primary (NPE data) or secondary (IE data) chambers. The N,, 0,, CO and Ar used in the studies were taken from commercially available cylinders and used without further purification. Mixtures of the gases were blended in the inlet system of the mass spectrometer. Under typical operating conditions, the pressure was estimated to be - 10S3 Torr within the sample gas beam being intercepted by the pulsed electron beam. During pressure dependence studies, the estimated pressures ranged from - 10m5 to - lop2 Torr. Pressure in the secondary chamber due to scattered sample gas was found to be about one-sixth that in the primary chamber. RESULTS

AND

DISCUSSION

All possible atomic ions were observed for the gases while operating the secondary chamber in the R-mode. The abundances of these ions were linearly dependent on the pressure and ionizing current in the primary chamber, hence gas phase collisional ionization was assumed negligible. Neutral product efficiency plots were recorded for each of the atomic ions from the pure gases, examples of which are shown in Fig. 2. The pressures for the data shown were the same for all the gases, - 2 x 10m4 Torr. The detection sensitivities were equal for all the plots, so relative abundances of the various species can be estimated directly from the figure. Only in the cases of N+ and Ar+ was a significant increase in ion current observed when the electron beam of the secondary chamber was turned on. Diatomic ions were observed in low abundance for the diatomic gases while operating in the R-mode. Their abundances were dependent on the square of the pressure, but linearly dependent on the ionizing current in the primary chamber. No additional ions were observed when the secondary chamber was operated in the N-mode. Data for NPE plots for the diatomic neutral species were collected only for primary electron energies less than 30 eV. Examples of these plots are shown in Fig. 3. Anomalies were observed in the plots of these low-abundance ions at electron energies greater than - 35 eV. These anomalies arose from at least two sources. When the potential of the electron repeller electrode closest to the primary ionization chamber (5, Fig. 1) was not sufficiently negative, pulses of electrons elastically scattered from the sample gas beam could be accelerated into the secondary ionization chamber where they ionized the ambient gas. When the repeller electrode was more negative

97

I,

“‘“UC7

secondary chamber ion

current

I 10

20

70 PR~~ARY4&AMB?R

EL%RON

80 ENERGY

90

I00

(eV)

Fig. 2. Plots of neutral product efficiency data for atomic ions colIected in the R-mode operation of the secondary chamber.

5 PRIMARY

IO

15 CHA~MBER

20 ELECTRON

25 ENERGY

30 kV)

Fig. 3. Plots of neutral product efficiency data for diatomic ions collected in the R-mode operation of the secondary chamber.

98

than - - 35 V, the leakage field into the primary chamber accelerated positive ions to the electrode. Neutral species were formed via charge exchange and their kinetic energy depended on the potential of the electronsuppressor electrode. Studies of binary mixtures of the gases in the R-mode of operation showed that the abundances of the atomic ions depend only on the partial pressure of the parent gas. However, the abundances of the molecular ions depend on the nature of the mixture as well as the product of the partial pressures. Nitrogen N’

Neutral product efficiency plots for N’ from N,, N-/N,, covering a 100 eV range of electron energies are shown in Fig. 4. The two plots were drawn with the same detection sensitivity while the secondary chamber was operated in the R-mode, lower plot, and the N-mode, upper plot. The lower plot is nearly identical to that reported by Kupriyanov [21] for the production of N + from Rydberg nitrogen atoms when these nitrogen atoms, (N’)**, come near a metal surface. As seen in Fig. 4, the threshold for the production of (N’)** is - 25 eV. Comparison of the threshold region for N+ observed in the secondary chamber with that for N+/N, observed in

COMPONENT SECONDARY CHAMBER

CURRENT

IO

20

40

30 PRIMARY

60

50 CHAMBER

70

ELECTRON

Fig. 4. Plots of neutral product efficiency operation of the secondary chamber.

80

ENERGY

data for N’/N,

tev)

measured

in the R- and N-modes

of

99

the primary chamber is shown in Fig. 5. Clearly, the data indicate that the energy required for the formation bf (N‘)** is about equal to, or only This evidence slightly less than, that required for the formation of N+/N,, supports the conclusion that the N’ observed in the secondary chamber (R-mode) is due to ionization of (N’)**. Experiments using the secondary chamber focus electrode, Fig. 1, as a retarding potential electrode showed that the kinetic energy of the N+ ion observed.in the R-mode was what would be expected if N+ were formed from (N’)** at the metal surfaces of the secondary chamber (or at the surface of the repeller grids when they were operated at potentials positive with respect to the secondary chamber)_ This observation agrees with Kupriyanov’s observations of Rydberg -atoms [20]. Thus, the N+ observed from the secondary chamber is due to neutral species excited to - 24.3 eV in the primary chamber and ionized in the secondary chamber according to reaction (1). N,

2

N'+

(N')**

Pfi

metal + set

N++

e-

(1)

A second upward deflection in the lower plot of Fig. 4 occurs at - 50 eV. This deflection is considered to be related to the onset of either of two

I

I

I

I

I

ION (PRIMARY

CURRENT

CH

UTO,SECONDARY

PRIMARY

CHAMBER

ELECTRONENERGY

leV>

Fig. 5. Plots of neutral product efficiency data for N’/N, N +/N, _

and ionization efficiency data for

100

additional reactions: the formation yields a doubly-charged molecule _

metal -+

+N;** pri

N;+

2

state (N;**)

2 e-

N++

e-+

pri

of an atomic ion and a Rydberg

(N’)**m?‘N++eset

which

(2)

SeC

or the co-formation N,

+

of a supraexcited

atom

(3)

However, since a 1 : 2 : 1 mixture of 14N14N, i4N1’N, and i5N15N yielded no ion at m/z = 14.5 (14N15N)‘+, even at a primary electron energy of 80 eV, reaction (3) is considered to be responsible for the second upward break in the R plot for N+. The additional ions observed while operating in the N-mode, upper plot of Fig. 4, were due to the ionization of atomic nitrogen by the 20 eV electron beam in the secondary chamber. Using the appearance energy of (N’)**/N, as a secondary standard, the appearance energy of N./N, (primary chamber) was found to be 9.9 + 0.7 eV. The large uncertainty is due to the low abundance of the N-/N, process. This observed energy value agrees with the known dissociation energy of N,, 9.8 eV. Thus the reaction _ N, 2 N’+ N’ > N++ e(4) Pfi

set

is responsible for the N + observed at primary chamber electron energies less than 25 eV. The greater noise on the upper plot of Fig. 4 is due to the 24 Hz component of random noise on the d-c. N + ion current generated by the 20 eV electrons in the secondary chamber. The choice of 20 eV electrons for the secondary chamber was a compromise between higher energy values to achieve greater sensitivity for N+/N’ and lower energy values to reduce the sensitivity for N+/N2_ (The high pressure of scattered N, in the secondary chamber and the energy spread in the electron beam results in comparatively large amounts of N+/N2 even for a nominal electron energy of 20 eV.) Ionization efficiency (IE) data collected from the secondary chamber and plotted in Fig. 6 verified that N-/N, is produced in the ground state. Molecular nitrogen was used to calibrate the energy scale. The ionization energy of N-/N, was found to be 14.3 + 0.2 eV, in excellent agreement with the known ionization energy of atomic nitrogen in the ground state 14.53 eV [323The abundance of ions in the energy region of the N-plot above 25 eV, Fig. 4, differs from that in the R-plot by a constant amount. Thus reaction

101

(4) is the principal source of ground state atomic nitrogen at all energies, in agreement with the observation of Niehaus [4].

N*

An NPE plot for N,+ is shown in Fig. 3. The threshold energy for the appearance of NC was found to be 10.3 -&0.5 eV in NPE expkriments where the threshold for (N’)**/N, was used to calibrate the energy scale. The variation of the abundance of N2+’ from the secondary chamber as the square of the pressure indicated that these ions were formed in gas phase collisions. The threshold value for N2* agrees fairly well with the energy of metastable N2(lrg), 8.58 eV [33]. Thus the reaction of metastable nitrogen, Nz, is proposed as the source of N,) at electron energies below the ionization energy, according to the reaction N;(r7rg) + N,*(‘T~)

*

set

N,+’ + N, + e-

(5)

Oxygen

The NPE plots for O/O, in the R-mode showed the threshold to occur at 19.3 + 0.5 eV with a second inflection at - 45 eV. Thus, analogous to the

SECONDARY CHAMBER ION CURRENT

SECONDARY

CHAMBER

ELECTRON

ENERGY

Fig. 6. Plots of ionization efficiency data for N+/N’ operation of the secondary chamber.

(e’v’)

and N */N,

collected in the N-mode of

102

observations _

for N-, the ions at threshold result from the reaction metal +

o$o+o**

pri

set

O+‘+ e-

(6)

and those producing the second inflection 0,

‘-

pri

o+*+

o**

metal +

result from

O+‘+ e-

set

No additional atomic ions were observed when the secondary electron beam was turned on (N-mode).

02

An NPE plot for 0: is shown in Fig. 3. The threshold of the NPE plot for OF was at 19 + 1 eV, the energy needed for the formation of 0**/02. Thus OT observed from the secondary chamber (R-mode) is the product of a collision between a Rydberg atom and a ground state molecule according to the reaction _ 0, “+ 0 + 0** 2 0;. + e-‘ (8) pri

set

Carbon monoxide C The threshold for the NPE plot of C occurred at 22.4 + 0.5 eV. Additional upward inflections are evident at 28.5 f 1.0 and 45 + 5 eV, Fig. 2. At the threshold energy, the source of C” ions is the reaction CO%

O+C**

Ph

metal

+

sec.

C+‘+e-

(9)

The energy of the second upward inflection is in reasonably good agreement with the value expected if two electrons in the carbon atom are excited, one of which is in a high Rydberg state. It is proposed that this supraexcited atom yields C”( 4P), excitation energy 5.3 eV [34], according to _

CO > 0 + C***

metal

+

set

(C+?* ’

’

+ e-

00)

The final upward inflection results from the reaction _ metal CO 2 O+‘+ e-+ C** + C+‘+ ePfi

the coformation

set

of an ion and a Rydberg

atom.

103

0 The ion O+‘/CO was the only ion in the study reported here which gave an NPE plot with an extended “foot” near threshold. The plot had curvature for - 5 eV from its onset at 26 + 2 eV. Presumably this extended foot results from the unfavorable energetics of formation of O**/CO metal -+ set

cok+o** pri

O++e-

(12)

compared with that of C**/CO, reaction (9), [IP(C) = 11.26, IP(0) = 13.62 eV] [32]. The sharper rise at - 30 eV, Fig. 2, probably results from the onset of the reaction _ ?q0+-)* + ecok+o*** 03) pri

set

similar to reaction (10) proposed above since the excitation energy of 0’.( ‘P”) is 5.0 eV [34]. The final upward inflection at 43 f 5 eV is considered to be due to _ CO

2

C+‘+

e-+

0**

metal

+

O+*+ e-

It is seen in Fig. 2 that O**/CO electron energies greater than 35 eV.

(14) is more

abundant

than C**/CO

at

co An NPE plot for CO+ is shown in Fig. 3. The threshold occurred at 8.4 + 0.3 eV. This value agrees fairly well with the energy of metastable CO( ‘A;), 7.57 eV [33]. Ionization resulting from the collision of two metastable species COc3Ai)

+ CO(3Ai)

+ CO+*+ CO + eset

is considered to be the source of CO+ at low electron energies. A second upward inflection in the NPE plot at 29 f 1 eV agrees reasonably well with the energy of the second upward inflection in the.O**/CO NPE plot, Fig. 2. Therefore a collision of 0*** with CO 0***

+ CO + CO+‘+ 0 + eset

(16)

provides an additional source of CO+ at higher electron energies. Argon Ar The threshold for the NPE plot for Ar was the same, within experimental error, as the ionization energy of ground-state argon, whether operating in

104

the R-mode or the N-mode (10 eV electrons in the secondary chamber). Thus, the excited species of argon observed at threshold was a Rydberg atom, Ar* *. The ions observed in the R-mode were produced according to _

Ar 2 Ar** PA

metal

+

set

Ar+‘-

+ e-

and the additional ions observed in the N-mode _ eAr 2 Ar** + Ar+‘+ ePA

set

(17) were produced

according

to (18)

At higher pressures, a quadratic dependence of the abundance of Ar+‘was observed in the R-mode. This quadratic dependence was studied more closely by taking NPE data over a wide range of pressures. The data at each of several primary chamber electron energies in the 18-50 eV range were found to give an excellent fit to the equation I+=

aP2 + bP

09)

where I+ is the observed Ar+‘current and P is the ion gage pressure of Ar. A plot of the coefficients b vs. electron energy was nearly identical in shape to the NPE plot for Ar** at low pressures. The coefficients a were negative at energies below - 35 eV and positive above 35 eV. This behavior above 35 eV for argon was not an instrumental artifact, as discussed previously, because the observation was made only for argon and was independent of the potentials on the repeller electrodes. We interpret these observations as follows: (1) bP is a measure of the production ,of the Ar** which can be ionized at metal surfaces; (2) for negative values of a, aP2 is a measure of the Ar** lost due to gas phase collisions occurring between the primary and secondary chambers; and (3) for positive values of a, aP2 is a measure of the Ar++’ produced by the gas phase collision of a supraexcited state of argon, Ar*** - 35 eV) with a ground-state argon atom. The first two (threshold conclusions are straightforward. The third conclusion is supported by the observation by Varga et al. [35] of metastable argon ions with a threshold of 30-35 eV. A pressure study of the increase in Ar+’ observed when operating in the N-mode resulted in data which also gave a good fit to Eq. (19) at all electron energies studied. The coefficients b were similar to those observed for the R-mode data. The coefficients a were negative throughout the 18-50 eV range studied. Thus, interpretations (1) and (2), above, apply in the electron energy range of this experiment. Electron ionization of Ar*** does not contribute appreciably to the increase in the observed Ar+’ current. The observation of negative coefficients a for the pressure dependence of Ar +’ indicates that the Ar** is more unstable to collision than are the other

105

Rydberg atoms studied. Presumably this is due to the greater internal energy of Ar** compared to (N’)*, 0*, and C*. The appearance energies of the neutral species observed in the title gases are summarized in Table 1. The relative abundances of the ions observed in the R-mode of operation for 30 eV primary chamber electrons are listed in Table 2. The data are for pure components obtained at the same ion source pressure, - 3 X 10T3 Torr. Binary gas mixtures Binary gas mixtures of approximately 1: 1 composition were introduced into the neutral fragment mass spectrometer and NPE data for all the possible ions were taken in the R-mode. The NPE plots for the atomic ions were identical in shape to those obtained for the pure gases. The abundances of the atomic ions were what was expected on the basis of the partial pressures. For the O,-CO mixture, the abundance of O**/O, was much

TABLE

1

Appearance energies of neutral species Appearance energy

Neutral species

(eV> CW**/N, NYN, NZP,

24.3 + 0.1; 45 + 5 9.9kO.7 10.3 f 0.5

o* */o,

19.3f0.5;

c**/co o* */co co*/co

22.4 f 0.5; 28.5 + 1.0; 45 + 5 24+2; - 30 8.4f0.3; 29fl

Ar**/Ar

15.7fO.l;

TABLE

45*5

37*3

2

Relative abundances of ions observed in R-mode of operation for 30 eV primary electrons and equal ion source pressures ( - 3 X 10e3 Torr) Ion

Abundance

N+/N, NT/N,

24 2

0 +--0, w/o,

100 8

Ion

Abundance

c+/co o-/co co+;/co

36 8 2

Ar +/Ar

30

106

greater than that of O**/CO. Since the threshold for O**/O, is also lower than that for O**/CO, the O+ NPE plot was essentially that of O**/02_ Ar-N,,

Ar-O,,

Ar-CO

In these mixtures, the abundance of the diatomic ions was more than ten times greater than what was expected on the basis of the partial pressures of the diatomic gases. The shapes of the NPE plots for N,+‘, 0; and CO+‘were nearly identical to that for Ar+: Thus, it is clear that (1) collisional ionization of these diatomic molecules was observed in the instrument at the pressures studied, (2) the diatomic gases were ionized by Ar**, and (3) Ar** is efficient in ionizing these diatomic gases [IP(Ar) = 15.76 eV, IP(N,) = 15.58 eV, IP(0,) = 12.06 eV, and IP(C0) = 14.01 eV] [32]. N2-02,

02-CO,

and CO-N,

Increases in abundances for all the diatomic ions were observed for these mixtures, compared with that expected on the basis of the partial pressures of the components. The increases were greatest for CO+’ in the O,-CO mixture, m/z = 28 in CO-N,, and 0: in N,--0,. The NPE data showed that, while interactions between Rydberg and ground-state species were important at higher primary electron energies, a substantial portion of the increase in diatomic ion abundances result from interactions of Rydberg and metastable species and metastable-metastable collisions. The NO+ ion was observed in the secondary chamber from the N,-0, mixture. The NPE data indicate that NO+ results from a (N‘)**-0, interaction. CONCLUSIONS

The neutral fragment mass spectrometer was found to provide information concerning the production of neutral species in highly excited states, as well as the ground state. The NPE data for atomic ions collected near the threshold energy were due to Rydberg states of the atom rather than autoionizing states. The broadly curving upward inflections at - 45 eV in the NPE plots for N+/N,, O+/OZ, C*,/CO, and O+*/CO, Fig. 2, occurred at electron energies higher than required for the production of two singly charged atoms (34, 33, 36, and 36 eV, respectively). The sharply rising portions probably result from contributions to the abundances due to Rydberg atoms which have a second electron in an excited orbit. The O**/O, was the most abundant of the Rydberg atoms studied, see Table 2. However, on the basis of the experiments with binary mixtures, Ar** was most reactive. The reactivities are related to the excitation energies

107

of the Rydberg atoms. The present neutral fragment mass spectrometer is limited to the kind of collision studies presented here, because no separate provision has been made for a collision gas. REFERENCES 1 G.D.

2 3 4 5 6 7 8

9 10 11 12 13 14 IS 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35

Flesch and H.J. Svec, Int. J. Mass Spectrom. Ion Phys., 38 (1981) 361. D. Beck and 0. Osberghaus, Z. Phys., 160 (1960) 406. D. Beck and A. Niehaus, J. Chem. Phys., 37 (1962) 2705. A. Niehaus, Z. Naturforsch. Teil A, 22 (1967) 690. C.E. Melton, J. Phys. Chem., 74 (1970) 582. J.R. Reeher, G.D. Flesch and H.J. Svec, Int. J. Mass Spectrom. Ion Phys., 19 (1976) 351. J.R. Reeher, G.D. Flesch and H.J. Svec, Org. Mass Spectrom., 11 (1976) 154. E.E. Muschlitz, Jr. and L. Goodman, J. Chem. Phys., 21 (1953) 2213. W. Lichten, J. Chem. Phys., 26 (1957) 306. W. Lichten, J. Chem. Phys., 37 (1962) 2152. J. Olmstead, III, AS. Newton and K. Street, Jr., J. Chem. Phys., 42 (1965) 2321. H.F. Winters, J. Chem. Phys., 43 (1965) 926. R. Clampitt and A.S. Newton, J. Chem. Phys., 50 (1969) 1997. R.S. Freund, J. Chem. Phys., 51 (1969) 1979. V. Cermti, J. Chem. Phys., 44 (1966) 1318. E.N. Lassettre and S.A. Francis, J. Chem. Phys., 40 (1964) 1232. V.D. Meyer and E.N. Lassettre, J. Chem. Phys., 44 (1966) 35. E.N. Lassettre, A. Skerbele and V.D. Meyer, J. Chem. Phys., 45 (1966) 3214. V. Cerm& and Z. Herman, Collect. Czech. Chern. Commun., 29 (1964) 953. SE. Kupriyanov, Zh. Eksp. Teor. Fiz. Pisma Red., 5 (1967) 245; JETP Lett., 5 (1967) 197. S.E. Kupriyanov, Zh. Eksp. Teor. Fiz., 55 (1968) 460; Sov. Phys. JETP, 28 (1969) 240. S.E. Kupriyanov, Zh. Eksp. Teor. Fiz., 56 (1969) 1524; Sov. Phys. JETP, 29 (1969) 818. H. Hotop and A. Niehaus, J. Chem. Phys., 47 (1967) 2506. H. Hotop and A. Niehaus, Z. Phys., 215 (1968) 395. T. Shibata, T. Fukuyama and K. Kuchitsu, Chem. Lett., (1974) 75. J.A. Schiavone, S.M. Tarr and R.S. Freund, J. Chem. Phys., 70 (1979) 4468. R.F. Stebbings, Science, 193 (1976) 537. B.M. Smirnov, Usp. Fiz. Nauk, 131 (1980) 577; Sov. Phys. Usp., 23 (1980) 450. SM. Tarr, J.A. Scbiavone and R.S. Freund, Phys. Rev. Lett., 44 (1980) 1660. S.M. Tarr, J.A. Schiavone and R.S. Freund, J. Chem. Phys., 74 (1981) 2869. G-D. Flesch, R.M. White and H.J. Svec, Int. J. Mass Spectrom. Ion Phys., 3 (1969) 339. H.M. Rosenstock, K. Draxl, B.W. Steiner and J.T. Herron, J. Phys. Chem. Ref. Data, 6, Suppl. 1, (1977). K.P. Huber and G. Herzberg, Molecular Spectra and Molecule Structure, Vol. IV, Van Nostrand Reinhold, New York, 1979. C.E. Moore, Atomic Energy Levels, Vol. 1, N.B.S. Circ. 467, 1949. P. Varga, W. Hofer and H. Winter, J. Phys. B, 14 (1981) 1341.