Journal of Electron Spectroscopy and Related Phenomena, 9 (1976) 419-439
@ Elsevier Scientific Publishing Company, Amsterdam - Printed in The Netherlands
PENNING IONIZATION ELECTRON AND OF TRIATOMIC MOLECULES
SPECTROSCOPY N,O, NO,, CO,,
OF CO, HCl, HBr COS AND CS,
J.Heyrovskj Znstituteof Physical Chemistry and Electrochemistry, Czechosbvak Academy of Sciences, Prague (Czechoslovakia)
(Received 1 March 1976)
ABSTRACT
The energy of electrons released in ionization of di- and triatomic molecules by He (2%, 23S), Ne and Ar (3P,, 2) metastable atoms was measured. Attention was concentrated on the determination of peak shifts and peak form in order to elucidate new features of the mechanism of ionization with respect to ionization of atoms. In ionization by He (2’S) atoms, asymmetric peak forms and greater shifts towards lower electron energy were found than in ionization by He (23S) atoms. A tentative explanation for peak shifts in terms of orientation of molecules during the collision and of molecular orbitals involved in ionization was proposed. Effects due to J values of metastable
atoms
and molecular
ions were
described.
The
ionization
of electro-
nlolecules (NO,) probably takes place already at large particle separation because of mixing with the X+ABCionic state which, on recombination, ionizes into X + ABC+ + e.
negative
INTRODUCrION
Since the fundamental aspects and theory of Penning ionization of atoms have been established ly 2 it seems appropriate to investigate the ionization of polyatomic molecules. The method used is the determination of the energy distribution of electrons released in ionization by metastable noble gas atoms (Penning ionization electron spectroscopy, PIES)‘5. In this paper, interest is concentrated on all features of the electron energy distribution where PIES differs from photoelectron spectroscopy, PES. Differences in the spectroscopies manifest themselves in shifts of electron energy peaks, in their shapes and broadness, and in the population of ionic states and their vibrational levels. In PIES, changes in peak shape and peak positions arise because the energy of released electrons is modified by weak interactions, in the state of collision, of
420 reactants, i.e. metastable atoms and target molecules on the one hand and of products, i.e. molecular ions and de-excited atoms in the ground state on the other” ‘, 6. The experimentally determined energy of electrons, released in Penning ionization, E,, is given by E, = E(X*) - [IP, (AB) = E, f AE
+ E,, j (AB’)]
& AE
(1)
where E(X*) is the excitation energy of metastable ionizing particles, IP, (AB) is the first or higher ionization potential of the molecule AB, E,, j (AB+) is the vibrational and rotational excitation of the AB+ ions and AE is the peak shift. The peak form and peak positions actually reflect the interplay of the main factors determining them: the shape of the potential energy surface of the reactants, V*(R), and products, Y* (R) and the transition frequency W (R)l’ 2. In ionization of Ar, Kr and N2 shifts to greater electron energy are found as an effect of transformation of the energy of relative motion of reactants into internal energy and/or formation of associated ions7-9. Shifts towards lower energy occur in ionization of H, alkali atoms and Hg due to ionizing transitions occurring at the attractive part of the potential energy curve for reactants77 lo’ ‘I. In both cases, the shift depends on the spin state of the metastable particles. In ionization by metastable 3PZ Ne and Ar atoms, smaller shifts are registered 8. Changes of the vibrational level population are connected with various from neutral resonant levels, changes of internuclear factors, e.g., autoionization distances in target molecules perturbed during the coIlision and/or with the vibrationaltranslational energy transfer in the collision complex after ionization”. The changes in population of ionic states with respect to photoionization may occur because of I the very different nature of the ionizing interaction of both heavy particles17 2.. Some of the phenomena which could be encountered when studying the Penning ionization of polyatomic molecules have been surveyed in a publication on the Penning ionization of molecules containing the CN groupI’. The investigation revealed that the greatest difference, with respect to PES, lies in the population of various ionic states and in the peak shift and broadness and their dependence on the nature of the metastable particle (He in the 2% and 23S states and Ne in the 3P2 state). Only shifts towards lower electron energy were found. The peaks due to ionization by He(21S) were always much broader and more shifted than those due to ionization by He(23S). In this study, the electron energy distribution curves were determined for ionization by He(2rS, 2’S), Ne(3P,, o) and Ar(3P,, o) atoms. The curves for Ar and CO are shown, too, in order to demonstrate how the relative peak heights and shapes change, in the same apparatus, when going from atoms and diatomic molecules to triatomic molecules.
421 EXPERIMENTAL
The Penning ionization was studied in an apparatus described in a previous publication’. The sampling system for introduction of the molecules into the collision chamber was doubled so that another gas, usually argon, could be introduced simultaneously into the collision chamber. This made it possible to check the resolution and also to verify how far the population of the He(2lS, 23S) atoms in the atomic beam in the collision chamber changed, by interaction outside the collision chamber, after introduction of molecules. The inside walls of the collision chamber were covered with benzene soot to suppress the stray electron background. No troublesome increase of the background current occurred even at electron energies smaller than 1 eV and no memory effects were observed. The pressure of the molecules on the TABLE 1 PEAK SHIFTS AND RATIOS OF AREAS UNDER THE PEAKS IN IONIZATION NEON AND HELIUM METASTABLES .Compound
State
OF ARGON,
Rb
Peak shifts (me V) a He* ~__ 21s
Ne* 23s
A?-*
3P2
3Po
3P.Z
3Po
Ar
=p3/2
35
35
1.7
co
=p1/2 x2x+
35 30
35 25
1.0
-
B%+ HCI HBr
N20 NOz co2
cos
csz
X 2II
A%+ X2I-I A2C+
XW A%-+ C?z+ 3BZ. (18.89 eV) X2& AZ% BY&+ C%g+
X 2II B%+ C%+ X2& A=% B2&+
-145 - 50 - 95
-
85
- 75 -100 0 -60
- 90 - 185 - 90 - 45 -190
-
-
15 50 15 50 30 15 15
0.37 0.32 30 15 0
0.90 0.80
0 - 25 -150 - 45 - 65 - 50 - 15 - 50 -120
15
0.90
20
0 5 - 35 -100 - 25
-45 -55
a Peak shifts are defined arbitrarily, as explained in the Experimental part. Probable error: & 5 meV. b Definition of R is given on p. 422.
-20
0.65 0.78 0.48 0.33
422 collision chamber was regulated to obtain the maximum counting rate and was usually of the order of lo- 5-10s4 mm Hg. The electron energy was held constant at 80 eV and the total electron current usually at 10 mA. Because a part of the 21.2 eV photons penetrated from the excitation region into the collision chamber, the measured spectrum contained small photoelectron peaks. They were helpful for comparison purposes and for calibration of the energy scale. The width at half maximum of the photoelectron argon peaks was 25 meV. No negative voltages were used to prevent stray electrons from penetrating into the collision region. Therefore, the beam of metastables was free of fast neutrals which might be formed by charge exchange outside the excitation region. The peak shifts may be defined arbitrarily as the distance of the peak maximum on the broadened asymmetrical peaks from the theoretical peak position at E,. If several broadened vibrational peaks merge to a broad band with no maximum, only band width can be defined as (1) the difference between E, and the energy E, corresponding to 44% of the peak height at the lower energy side* or (2) as the energy width of the base of the band counted from E, according, e.g., to Fig. 13. In this work, peak shifts are measured and the data are summarized in Table 1. Because the greatest emphasis is given to peak shifts and peak shape, no vibrational energies are tabulated or compared with photoelectron measurements. All of these data can be read from Figs. l-20, when needed. RESULTS
Argon In order to demonstrate the energy resolution, to determine the ratio of densities of He(2lS) and He(23S) atoms in the beam of metastables in the collision region and to check the ability of the apparatus to detect the slight asymmetry of the peaks, the electron energy spectrum was registered at the beginning of the measurements (Fig. 1). In it the greater broadening of the He(23S) peaks and the shifts of the center of the peaks towards greater electron energy, as well as the asymmetry of the He(2’S) peaks, are clearly visible. The ratio of the areas under the 2lS and 23S peaks, R, has the value of 1.7. In this case R = n, 0,/n, ot where n, and n, are the densities of the singlet and triplet metastables averaged over the quasi-thermal velocity distribution and us and 6, are the similarly averaged ionization cross-sections, Carbon monoxide The electron-energy distribution curve for ionization of CO by He(2lS, 23S) atoms into the X2X+ and B2YL+CO+ states is shown in Fig. 2. Although the densities
* The shifts measured in this way determine, in atomic systems and under suitable conditions, the depth of the minimum on the potential energy curves. The definition was proposed by Miller et al. for atomic systems and is suited if the peak is a true Airy peak19 2, 14.
423
He*-Ar 15.EeV
hv
A.0 E,(eV)
5.0
Figure 1. Electron energy distribution curve (EEDC) for ionization of Ar by 21.2 eV photons and He metastables. Nominal energies of electrons, EO (eqn. (l), p. 420), are indicated by vertical arrows.
He*-CO
i 1
I
1.0 Figure 2. EEDC
for ionization
of He(2lS) and under the peaks 2lS peak of the electron energy) first 23S peak is
I
0.5 E&J)
of CO.
He(23S) atoms in the beam did not change, the ratio of the areas of ionization into the CO+ ground state decreased to R = 1.0. The X2X+ ground state is shifted to a negative AE value (towards lower but that of the B%+ state is shifted to positive valuesi2* 15. The visibly assymmetric.
Hydrogen chloride In ionization of HCl by He(2lS, 23S) atoms into the X’II HClf ground state, relatively large negative shifts occur (Fig. 3). The 2’S peak is visibly asymmetric and
424
He*-HCI I.Bl2.75eV
hv
-1L5meV -I I-
8.5
7.5
8.0
7.0
E,
6.W
Figure 3. EEDC for ionization of HCl into the HCl+ X211 ground state. He*-HHCL
1
I
I
5.0
4.0
3.0
E,feV)
Figure 4. EEDC for ionization of HCl into the HCI+ A2C+ first excited state.
stretches to lower electron energy. The shift is again greater for ionization by He(2’S) than by He(23S) atoms. In ionization into the first excited A2C+ -state, the shifts are negative, too, but much less so (Fig. 4). The vibrational distribution in this band has been discussed in two previous papers ’ 6 ’ 1 7. The ratio of areas under the 2l S and 23S peaks of the first band decreased to R = 0.35 while that for simultaneouslymeasured peaks of Ar decreased to R = 0.9. Hydrogen bromide
In ionization of HBr into the X’II ground state and A2X+ HBr+ first-excited state by Ne(3P,), atom shifts towards greater electron energy occur (Fig. 5). If He(2%, 23S) atoms are used, the peak shifts become negative in both X’II and A2X+ bands (Figs. 6 and 7). The 2% peaks of the A%+ band are very broad and
425 Ne*- HEr
, Figure 5. EEDC stables.
9.5 Figure 6. EEDC
for ionization
9.0
for ionization
1.5
1.0
0.5E,leV)
of HBr into the HBr+ ground and first excited
8.5
8.0
7.5&J
states by Ne meta-
E,
of HBr into the HBr+ ground state.
unresolved. The ratio R is decreased to 0.32. The differences of the population of the vibrational levels of the A%' state in Figs. 5-7 were discussed previously”. According to Fig. 5, the shift of the 3P, peak for ionization into the HBr+ X’KI,,, state is probably negative because it merges into the 3P2 peak and is not resolved at all.
426
HeLHBr
I
I
I
6.0
5.5
5.0
-3b’mcV I
I
L.5
4.0
I
3.5 Ee(eVl
for ionization of HBr into the HBr+ first excited state.
Figure 7. EEDC
I Ne*- N,O
A.0
I 3.5
/, I
HeT- N,O
I
Figure 8. EEDC for ionization
I 8.0
I
I 7.0
of NaO by Ne and He metastables
EJeV)
’
into the NzO+
ground
state.
Nib-0 us oxide
The electron-energy distribution curves for ionization by helium metastables into the X2TI, A2C-+ and C2EfN20+ states and by neon metastables into the X211 state are shown in Figs. 8 and 9. The neon 3P 2, O peaks are not shifted at all but shifts towards negative AE values occur in ionization by helium metastabIes with the exception of the 23S peak of the A2C’ state. The shifts of the 2lS peaks are always greater than those of the 23S peaks.
427
He*-NNZO
A
4.0 Figure 9. EEDC
for ionization of N2O into the excited NzO+ states indicated.
Nitrogen dioxide In ionization of NO, all peaks besides those belonging to ionization into the 3B, excited NO: state (IP = 18.86 eV) are low and are not shown here. The shifts of the peak of the 38, band (and also of the ‘A, band, IP = 13.60 eV) are zero (Fig. 10). This is very surprising. Rather strong negative shifts should be expected in this case the more so because metastable helium behaves, in scattering experiments, like Li atoms *.
Carbon dioxide The electron-energy distribution curve ion ionization of CO, into the X’II, ground state by Ne(3P,, *) a t oms is shown in Fig. Il. Many more vibrational peaks appear than in ionization by 21.2 eV photons, doubtless because of autoionization from a neutral excited state in resonance with the energy of the Ne(3P,) atoms. Similar autoionization has been reported in ionization by photons with the energy of 16.847 eV1’. A tentative assignment of the vibrational peaks as well as the peak shifts are shown in Fig. 11, too. The peaks of ionization into the X’IT,CO,+ ground state by He(2lS, 23S) atoms are shown in Fig. 12. The 2lS peak is distinctly asymmetric and is stretched to lower energy. Peaks of ionization into the A21Yl, by He(2’S) atoms are small and strongly mixed with photoionization peaks. In ionization into the same state by He(23S) atoms, the population of vibrational levels differs substantially from that l In a collision of Cs with NO2 a sticky collision complex is formed which yields CsO + NO as products’*.
428 He*- NO,
3B2 18.86eV
2.0 Figure 10. EEDC
1.0 for ionization
E,W
0
of NO2 into the NO Z+ 3B~ excited state with IP = 18.86 eV.
Ne*-CO,
Figure 11. EEDC for ionization of CO2 into the COZ+ ground state by Ne metastables. assignment of the vibrational peaks is indicated.
The tentative
known from photoelectron spectra (Fig. 13). The 000 level is the most populated one. The peak of ionization into the C2Cl state by He(2lS) atoms has a surprisingly steep rise. Carbonyl sulphide
Electron-energy distribution curves for ionization by means of neon and helium metastables are shown in Figs. 14-17. The ionization by He(2%, 23S) atoms gives rise to new features. These are, the overall shape of the peak of ionization by
429
6.0 E&V)
7.0 Figure
12. EEDC
for ionization
of COZ into the COz+ ground state.
He*-CO,
A%" 17.32 ev 2%
2’S
“”
mev / 350, I
2.0
3.0
Figure
13. EEDC
for ionization
jk .,mFd
1.0
e&k&
E&V)
,
0
of CO2 into the indicated COZ+ excited states.
He(2’S) into the C?Z+ COS+ excited state (which resembles that of ionization into the same state of CO,) and small peaks at electron energies around and smaller than 1 eV (Fig. 17). Peaks at I?, = 0.5, 0.77 and 0.87 eV can be ascribed to autoionization of excited 0* atoms, formed in dissociation of highly excited COS molecules into the CS + O* pair, after a resonant energy transfer from helium metastables. The peak at E, = 0.6 eV may be due to autoionization of the S* atoms in the
430
N e’-
1 5.5 Figure
I
5.0
14. EEDC
COS
I
“2.0
1.o
1 E,(eV)
0
for ionization of COS into the COS+ ground and excited states by Ne metastables.
3~~3p~(~D)5p’(‘~
3F’,
3D,
“P)
and
3~~3p~(~D)6s’(‘~ “D)
states
according
to
the
process He*
+ COS S”
The peak at E, 3s23p3(2P)5s”(l’ Carbon
= CO + S* = s+(%) + e
= 0.95 eV may be due to autoionization “P) states into the Sf(4S) ions*.
(2) of the S* atoms in the
disulphide
In ionization of CS, by Ar(3P2, o) atoms, both X2J13,2 and X2111,2 peaks are resolved and their heights can be measured (Fig. 18). Contrary to ionization by 21.2 eV photons, in ionization by Ar(3P,) atoms the cross-section for ionization into the 2rI 3,2 state is greater than into the 2II1,2 state. In ionization by neon metastables, the 3P,, peak is shifted more negatively than the 3P, peak. The same applies for ionization into the B2E,t state (Fig. 19). The ionization by He(2lS) into the X211, ground state is remarkable because the ratio under the 2lS and 23S peaks is only 0.33 even when the same ratio of areas under the argon peaks changes only slightly after introduction * Term and electron energies were calculated using known data given by MoorezO and the
procedure
outlined previously”1-23.
431
10.0 Figure
15. EEDC
8.0 E&V)
9.0
for ionization
of COS into the COS+ ground
He”-COS B2E;,
state.
d
16.04dr hv
I Figure
16. EEDC
for ionization
17.96ev
hv
into the indicated COS+ excited states.
of CS2 into the collision region (Fig. 20). Ionization into other CS: detected but the peaks were small and are not shown here.
states was also
DISCUSSION
In surveying the data obtained in ionization by He(2%, 23S) atoms from the point of view of particle interactions, several trends become apparent: (1) the peak
432 He-
COS
E,teVI Figure 17. EEDC for ionization sensitivity two times higher.
Figure 18. EEDC
0
of COS into the indicated COSI- excited states. Curve 2, the
for ionization of CSZ into the CS 2+ ground state by Ar metastables.
433
I
6.’
7.0
,‘,,
4.0
Figure
19. EEDC for ionization
11.0
3.5
I
of CSS into the C&f ground
10.0
1
I
2.0
1.5 E,W~
and excited states by Ne metastables.
E,(eV)
LO
Figure 20. EEDC
for ionization of CSS into the CSz+ ground state by helium metastables. Curve 1, of Ar admitted aione into the collision region. Curve 2, electron energy spectrum of Ar after simultaneous introduction of Ar and CS2 into the collision region. The pressures of CSZ and Ar in the collision region are the same as when the compounds are introduced alone.
efectron
spectrum
shifts in ionization of HCl and HBr and of triatomic molecules become negative (with the exception of NO2 molecules); (2) the 23S peaks are broadened, but they are symmetrical, or their asymmetry is so small that it cannot be seen under experimental conditions used; (3) the 2lS peaks are much more broadened and are asymmetrical with a wing extending to lower electron energy; (4) the negative shifts are greater for ionization by He(2’S) than by He(23S) atoms; (5) the shifts depend on the kind of
434 metastable atom. They are greatest with helium metastables and lowest when argon metastables are used. These characteristics of ionization were found already in ionization of molecules containing the CN group13 and are retained also in ionization of other CzH4, butadiene, methylpolyatomic molecules studied (H,O, H,S, SOZ, C,H,, acetylene and allene) 24y ” . Table 1 shows that negative shift appears already in ionization of CO into the X2Z+ ground state by He(2lS) atoms. The shift is - 30 meV. In ionization of HCl into the X2fI state the shift is - 145 meV and - 50 meV for ionization by He(2lS) and He(23S), respectively. In ionization of Hg the peaks are shifted to lower energy, too, and the 2’S peaks are slightly asymmetrical’. These facts can be classified if the shapes of the potential energy surfaces (or curves) of reactants are considered. The quasi molecules formed by combination of He(2lS) atoms with molecules in the singlet ground state are singlets, with the He(23S) they are triplets. It follows that the singlet surfaces must have deeper minima than the triplet surfaces. This conclusion runs parallel with other findings and with theoretical calculations for atomic systems’ 6 - ’ *. For example, the potential energy curves for alkali dimers, made by combination of two atoms in the ‘S ground state, are singlets and triplets. It was found that the singlet curves are more attractive than the triplet ones. The minima for singlet curves lie at smaller particle separations. If the target atoms are doublets (H, alkali atoms) the quasi molecules formed in the state of collision can be in the doublet state if the ionizing particle is the He(2’S) atom or in the doublet and quartet states if it is the triplet He(23S) atom. Because the quartet state cannot autoionize into the final doublet state (ion + electron), only doublet states need be considered. One is inclined to explain other findings in Penning ionization, besides those of H and alkali atoms, also by the shape of the potential energy curves: those doublet curves, in which the triplet helium metastables participate, must have deeper minima than other doublet curves with participation of singlet helium metastabtes. This classification serves, of course, only for orientation purposes. To check it one should compare the shifts in ionization of H and alkali atoms (which are doublets in the ground state) with the shifts in ionization of alkaline earth atoms whose ground states are singlets. The only case studied so far is the ionization of Ba(lS) by He(2lS, 23S) atomsz9. As anticipa ted, the negative shifts are smaller in ionization by He(23S) than by He(2’S) atoms. Ba atoms behave in this way as Hg or singlet state polyatomic molecules. To check the shifts in the pair which gives the collision complex in a doublet state, one should study the ionization of other particles with the doublet ground state, e.g. Al (“P) or free radicals NO t2f’I), NO2 (‘A,), ClO, (2S,) or other radicals like NF,. In the ionization of NO into the X1x+ ground state, the shift is slightly positive for He(2iS) (7 + 20 meV) and slightly negative for He(23S) (-9 f 15 meV), i.e. the difference lies in the anticipated direction but the results are not very conclusive’. Experiments with NO, revealed zero shifts, probably because the ionization takes place at very great interparticle distances.
435 Another classification concerning the magnitude of the shifts in ionization with the same metastables into various ionic states of the same molecule is possible. A striking example is the ionization of HCl into the X’II and A%+ states or ionization of CO2 and COS into the X’II and C2Z+ states by He(2rS) atoms (Table 1). The differences amount to 100 meV in the case of HCl and 120 meV and 100 mcV in the cases of CO, and COS, respectively. Because there is only one upper potential energy surface for the He(2rS) + HCl pair, the differences in peak shifts might be explained, assuming that they reflect the different shapes of the lower, ionic surfaces for the He{ 1 IS) + HClf (X’II) and He(1 IS) + HCl+(A’Z+) pairs, by reference to their repulsive parts. On the other hand, it might be even better explained by assuming that it is due to ionizations in two different ranges of particle separations, depending on the type of orbitals from which the electrons are removed. Because the interaction energy depends on the particle separation, the resulting negative shifts must be different. The differences in shifts may also result when different orientation of the polyatomic molecule is needed for ionization to take place with appreciable probability from different molecular orbitals. In such a case, shifts result when molecules possess permanent dipoles. This explanation is well suited for ionization of HCl and HBr. The X’IlHCl+ state results when an electron is removed from the 3p, and 3p, degenerate orbitals located on the Cl atom. Ionization probably occurs when the metastable He atom approaches the Cl side of the molecule; then the induction energy may become appreciable and greater negative shifts result than in ionization into the A2Z+ state, where an oriented approach is probably not needed. Further explanation is given to the shifts encountered with different metastable atoms. One should expect that in ionization by Art3P,) atoms the shifts will be as great or greater than those in ionization by He(23S) because the polarizability of the Ar(3P2) atom is greater than that of the He(23S) atom3’. In fact, in the series He*Ne*-Ar*, argon metastables produce smallest shifts. This effect is probably connected with the increased van der Waals radius of the Ar(3P2) atoms, which prevents them from approaching the targets as closely as the He(23S) atoms can. The lowering of the ratio of areas under the 2lS and 23S peaks can plausibly be explained assuming that in ionization of polyatomic molecules a parallel process takes place, i.e., the dissociation of molecules, excited by a resonant energy transfer, into neutral fragments. The cross-section for such a process should be relatively high, especially in HCl, HBr and CS2, and should depend on the spin state of the metastable atoms. Evidence by optical emission measurements of these dissociations has been given by Richardson and Setser 31 _ However, the lowering could also result if the angular distributions of electrons in ionization of polyatomic molecules by 2rS and 23S helium atoms were different. The effect of different shifting of peaks, depending on the total angular momentum of the metastabIe atoms and the final ionic states of the molecules (Fig. 5), has already been found for the ionization of Kr and Xe’. It is not restricted to ionization into states with different Jvalues; it also appears in ionization of CS2 into
436 the B22=,+ state (Fig. 19). This effect should be distinguished from another one bearing upon the relative heights of the peaks, as in ionization by Ar(3P,, 0) into ionic states differing in their J values (Fig. 18). An analogous effect was described in ionization of CH,15. Both effects are easily reproducible. Angular distribution measurement should probably be made before the nature of the phenomenon is elucidated. A further problem concerns the shape of the peaks and the significance of peak shift values. The asymmetric 2iS peaks can be due to ionizing transitions from the upper attractive potential energy surfaces vertically down onto the nearly nonattractive ionic surfaces with a probability which must be appreciable already at great particle separations and does not increase too steeply with decreasing distance. In this case, a maximum can appear which is not located at the smallest electron energies and which is, therefore, not an Airy peak ’ 3 2, 32. The symmetric or nearly symmetric 23S peaks are difficult to explain. Because the shifts are small and the peaks narrow, their shape cannot be determined exactly. Nevertheless, they are certainly more symmetric than the 2rS peaks of Ar (Fig. 1) or the 23S peak of CO+ in the X2Ec+ state (Fig. 2). A definite answer can be given only after the theory of Penning ionization of molecules is worked out. The present lack of this theory also invalidates the interpretation of peak shifts in terms of the depth of the minima on the potential energy surfaces of the reactants. It seems that a promising system for the future calculation of the molecular interaction energy is the pair He(2’S, 23S) + HCl. The calculations with the He atom in the ground state have already been published33. Finally, a note on possible mechanisms of ionization of molecules in contrast with those of ionization of atoms is appropriate. The ionization of atoms is adequately described by autoionization of the collision complex, without the need ‘of curve-crossing considerations. However, when the targets become molecular, the. potential energy surface of a combination of a metastable atom and a molecule may go over, by avoided crossing, to a number of surfaces of right symmetry and multiplicity belonging to a combination of the de-excited atom and a molecule in a highly excited Rydberg state 31 . If this occurs, the ionization probability is determined by the iifetime of the autoionizing Rydberg state, which is probably independent of the particle separation. Negatively-shifted and symmetric peaks should result. The ionization proceeds in this case through a direct mechanism, i.e., the released electron originates from the excited molecule. The energy transfer is of the type He*
+ ABC ABC*
+ He(llS) + ABC+
+ ABC* + c
It might be assisted by pseudocrossings He*
+ ABC
+ He+ABC-
+ Ekin
(2)
with the ion pair surface
+ He(l’S)
+ ABC*
(3)
according to Fig. 21. But, if the avoided crossing mechanism is operative and ABC* states are formed, one should expect that the population of vibrational levels
437 25.0~ 4
He++ABC-
Figure 21. Schematic potential energy curves for reactants. Curve 1, potential energy curve for the He(23S)+ABC pair. The attractive part of the curve is greatly exaggerated. Curve 2, one of the potential energy curves for the He(1 lS)+ABC* pair. Curve 3, potential energy curve for the Coulomb interaction of the He++ NC%- pair. If polarization potential were considered the crossing point, Rc, would shift to greater R. Curve 4, potential energy curve for Coulomb interaction of He++ ABCpair. EA ABC = 0.5 eV. Curve 5, potential energy curve for products. Hatched regions (I///) schematic location of ABC* Rydberg states with energy lower or equal to that of He(2”S). (\\\\)ionization continuum for the He++ NOz- pair.
of the ABC+ ions will generally be altered with respect to photoionization. Experimental data in this work show that, in most cases, the populationis Franck-Condonlike. Consequently, reactions (2) and (3) are not very probable in ionization
+ e) - + He
(ABC+
+ e) --f ABC+
+ (ABC+
+ e)
(4)
-I- e
(5)
In (4) and (5) the symbol (ABC + e)- represents schematically the negative ion ABC-. The energy of released electrons is given by (6), which is analogous to eqn. (1) E, = RE (He+)
-
EA (ABC)
-
[IP (ABC)
+ E (ABC’)]
-
AE
(6)
438 where RE is the recombination energy, EA is the electron affinity of the ABC molecule and AE is the lowering of the potential energy of the He+ + ABC- pair in the actual ionization distance R. The outlined mechanism is well suited to explaining the experimental results in the ionization of NOz and probably also of other highly-electronegative molecules. The electron affinity of NO, is 2.36 + 0.1 eV 34 . Therefore, the dissociation limit of the He+NO; surface (22.22 eV> lies relatively close to the assymptotic energy of the He*-NO, surface (19.81 eV for He(23S) and 20.61 eV for He(2lS) atoms) and the crossing of both surfaces occurs at very great particle separation (Fig. 21). If the probability of ionization upon recombination of the He+ + NO; ions (reactions (4) and (5)) becomes already appreciable at R > R, (Fig. 21), then the He*-NO, system will be ionized to the extent to which its wave function acquires the ionic character, i.e., very near and at the crossing point R,. At this separation, both potential energy surfaces of reactants and products are not modified by particle interaction_ In the vertical downward transition, electrons with nominal energy E, (eqn. (1)) will be released and, consequently, zero shifts will be found, as is actually the case. Of course, PIES is unable to prove that this mechanism is correct. Zero shifts in ionization of NO, would result if the transition probability, W(R), were, for some reason, approximately constant over the whole range of particle separations”. An insight into the problem would be obtained if reactions (4) and (5) were studied directly in recombination of suitable pairs of positive and negative ions.
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