J. Phys. Chem. 1994,98, 10277-10281
10277
Electron-Stimulated Desorption of H- Ions via Dissociative Electron Attachment in Condensed Methanol L. Parenteau, J.-P. Jay-Gerin, and L. Sanche*$t Groupe du Conseil de Recherches Mkdicales du Canada en Sciences des Radiations et Dkpartement de Midecine Nucltaire et de Radiobiologie, Facultk de Mkdecine, Universitk de Sherbrooke, Sherbrooke, Quebec JlH 5N4, Canada Received: May 11, 1994; In Final Form: July 7, 1994@
Low-energy (0-20 eV) electron impact on condensed methanol films is observed to induce electron-stimulated desorption (ESD) of H- ions, via dissociative electron attachment (DEA). The onset for H- detection is at 6 eV, with a maximum yield centered near 8.7 eV and a shoulder at 7.3 eV. We also observe a gradual increase in H- intensity above 12 eV, which is attributed to the dipolar dissociation mechanism. Comparison of ESD data on isotopically labeled methanols CH30D and CD3OH allows us to identify the originating sites of the H- ions. In contrast to the gas phase where the H- ions are known to arise exclusively from the OH group below 9 eV, the desorbing H-ions for condensed methanol are shown to originate in both the hydroxyl and the methyl groups. The release of H- ions from the hydroxyl group is shown to involve dissociative Rydberg anion states, in close similarity with ESD data of H- formation in amorphous ice. The kinetic energy distributions of the desorbing D- ions for both CD30H and CH30D are also reported. It is found that multiple electron scattering processes in the methanol films prior to DEA events enlarge the width of the peaks in the yield functions and that post-dissociation collisions at or near the surface may be involved in the reduction of the kinetic energy of the escaping anions.
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Introduction
The study of low-energy electron-molecule interactions is of central importance in a variety of fields such as surface reactivity,' plasma physics,2 dielectric aging,3 and radiationinduced degradation of organic and biological material^.^ The degradation processes induced by low-energy electrons in water and methanol in the condensed phase are of particular interest in the field of radiation chemistry, as they provide unique information about primary chemical species that are formed by secondary electrons in these solvents. In the energy range below the threshold for dipolar dissociation (DD), the dissociative electron attachment (DEA) to molecules (e.g., for a diatomic molecule AB, eAB [AB]A B-)*516 has been recognized as a major mechanism leading to stable anion and free-radical formation and to species-specific degradation processes." DEA consists of a resonant electron capture into a relatively short-lived molecular anion state, which is dissociative in the Franck-Condon region. When the lifetime of this state is similar to, or longer than, the vibrational period of nuclear motions, the transient anion can dissociate into neutral and anion fragments if at least one of these has a positive electron affinity. When DEA occurs at or near the surface of a solid, only anion fragments having sufficient kinetic energy (KE) to overcome the induced electronicpolarization potential can leave the surface to produce an electron-stimulated-desorption (ESD) signal. Previous studies of DEA in condensed-phase systems have included the effects of anion-induced polarization interactions? multiple electron scattering,8charge and energy transfer states?Jo perturbations introduced by changes of the target site symmetry and number density,' and post-dissociationinteractions of DEA anion fragments with adjacent molecules.12 DD (e.g., for a diatomic molecule AB, eAB [AB]* eA+ B-
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* To whom correspondence should be addressed.
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Also a member of the Canadian Centre of Excellence in Molecular and Interfacial Dynamics. Abstract published in Advance ACS Abstracts, September 1, 1994. @
e-) is another mechanism that produces ESD of anions at low electron energy. This nonresonant mechanism involves an intermediate electronically excited state (e.g., [AB]*) and only occurs beyond an energy threshold which necessarily lies above the dissociation energy of the fragments ( e g . , A+ B-). In this paper, we report the observation of DEA-induced ESD of H- and D- from multilayer films of pure methanol (CH3OH) and of two different deuterated methanols (CD30H and CH30D) in the electron energy range 0-20 eV. In studying ESD on isotopically labeled methanols, our goal is to elucidate the decomposition mechanism of the transient negative parent ion formed on electron attachment in the condensed phase. In order to isolate the intrinsic and extrinsic effects of the condensed phase on the DEA process due to the presence of similar neighboring molecules in the pure solid,13 we also present the D- desorption yield from the deuterated compounds physisorbed on multilayer krypton film substrates. In addition, KE distributions of the departing D- ions are reported for both CD30H and CH30D.
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Experiment
The basic features of the apparatus and its operating modes have been described in detail previously.l0 Briefly, the apparatus consists of a home-built hemispherical electron monochromator, a quadrupole mass spectrometer, and an electrically isolated polycrystalline platinum ribbon press fitted on the cold end of a closed-cycle cryostat. Multilayer films of methanol (CH30H. CD30H, and CH30D) molecules and rare-gas (Kr) atoms are grown on the ribbon which is held at a temperature of 17 K. All components are housed in an ultrahigh-vacuum system with a base pressure of -2 x Torr. An electron beam, with incident energies (EJ in the range 0-20 eV, an energy dispersion with a full width at half-maximum (fwhm) of -100 meV, and a current of typically 4 x A, impinges onto the target films at 70 from the surface normal. E, is calibrated within &0.4 eV with respect to the vacuum level, as O
0022-365419412098-10277$04.5010 0 1994 American Chemical Society
Parenteau et al.
10278 J. Phys. Chem., Vol. 98, No. 40, 1994
a,
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I
m
0 a,
z 0 0
l
1 2
4
6
8
0
10 12 14 16 18 20
Electron Energy (eV) Figure 1. Yield distributions for H- desorption from electron impact on a 5-monolayer (ML)-thick film of CHsOH deposited on the platinum substrate. The zero of energy corresponds to the vacuum level. The temperature of deposition is 17 K. previously described.l0 A portion of the ESD negative ion flux is focused by electrostatic lenses into the mass spectrometer. Two grids are positioned at the entrance of the mass spectrometer in order to measure the anion KE distributions by the application of a 0-5 eV retarding potential. The apparatus can be operated in two modes: (i) the anion yield mode in which negative ions of a selected mass are detected as a function of Ei and (ii) the anion energy mode in which the negative-ion current at a selected mass is measured for a fixed incident electron energy as a function of the retarding potential. In the present experiment, the deposited CH3OH, CD30H, CH30D, and Kr film thicknesses were determined with an estimated uncertainty of 50% by means of a previously described gas volume expansion pr~cedure.'~The stated purity of CH3OH was better than 99% with a water contamination of less than 0.005%. CD30H and CH30D were supplied by Aldrich at a stated isotopic purity of 99% and 99.5%, respectively. The stated purity of Kr was 99.995%. All samples were degassed in situ. It was not possible, for the deuterated methanols, to independently verify the extent of isotope purity. Because of the mass dependence of anion transmission in the mass spectrometer, the relative intensity between the H- and D- ESD yields could not be compared quantitatively.
Results The H- ion ESD yield obtained from a 5-monolayer (ML) film of CH30H deposited on the platinum substrate is shown in Figure 1 as a function of Ei in the range 0-20 eV. The Hyield has an onset at Ei = 6.0 eV, a maximum centered near 8.7 eV with a fwhm of 4.1 eV, and a shoulder at 7.3 eV. Figures 2 and 3 show the H- and D- ion desorption yields produced from electron impact on 5-ML films of CH30D and CD30H, respectively. The H- yield in Figure 2 has an onset at 6.0 eV and a maximum at 9.3 eV, with a shoulder near 7.2 eV. In the case of D- desorption, the onset is observed at 5.9 eV, and the D- yield exhibits a broad feature which appears to be composed of two overlapping peaks located at 7.2 and 8.6 eV. The Hand D- ion ESD yields for condensed CD30H (Figure 3) exhibit essentially the same behaviors as a function of Ei as those observed for the corresponding hydroxyl and methyl groups in CH30D (Figure 2). The only difference is that in Figure 3 the
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8
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Electron Energy (eV) Figure 2. Yield distributionsfor H- and D- desorptions from electron impact on a 5-ML-thick film of CH30D deposited on the platinum substrate.
CD,OH c
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4
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z
0
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Electron Energy (eV) Figure 3. Yield distributionsfor H- and D- desorptions from electron impact on a 5-ML-thick film of CD30H deposited on the platinum substrate. D- ions coming from the methyl group have a stronger contribution in the 8-eV region, the maximum for the Dformation being slightly lowered to 8.8 eV and the onset taking place at 6.1 eV. The gradual increase in anion intensity above -12 eV is attributed to the DD mechanism. The H- (D-) signal from CH30D (CD30H) generated via DD at Ei = 20 eV is about 4 times greater than that produced by the DEA process. In contrast, the yields of D- (H-) ions coming from the hydroxyl group have DD and DEA contributions of comparable magnitude (see Figures 2 and 3). No signal was detected for higher masses under the same conditions of current and accumulation time. Figure 4 shows the D- ion ESD yield measured from submonolayer (-0.1 ML) coverages of CH30D (the lower curve) and CD30H (the upper curve) on 20-ML films of Kr as a function of E,in the range 0- 12 eV. The H- ions were not recorded to avoid any contribution in the signal from water contamination. The two D- yield functions are slightly different depending on the originating site of the D- ions. Both anion
Electron-Stimulated Desorption of H- Ions
J. Phys. Chem., Vol. 98, No. 40, 1994 10279
Deposition
on Kr substrates
9 a
F n
CD,OH
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w CH,OD
0
2
4
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1
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Electron Energy (eV) Figure 4. D- yield distributions from electron impact on a 0.1-ML film of CH3OD and CD3OH deposited onto 20-ML-thick films of Kr.
0
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3
Relative Kinetic Energy (eV) Figure 6. Kinetic energy distribution for D- desorbing from a 5-MLthick film of CD30H. The incident electron energies are 8 and 9.5 eV.
0
1
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Relative Kinetic Energy (eV) Figure 5. Kinetic energy distribution for D- desorbing from a 5-MLthick film of CH30D. The incident electron energies are 8 and 9.5 eV.
yields exhibit three desorption maxima as a function of Ei with an onset near 6 eV. These maxima are better resolved in the case of CH30D, where they are observed at Ei = 6.9, 8.7, and 9.6 eV. The narrow peak at 9.6 eV is also clearly visible in the D- yield from CD30H. In both cases, the intensity ratio for the first two D- yield structures is sensitive to the quantity of methanol deposited on the Kr substrates. The relative KE distributions of the D- ions desorbing from 5-ML-thick films of CH30D and CD30H are shown in Figures 5 and 6, respectively, for incident electron energies of 8.0 and 9.5 eV. The zero of energy is taken as the difference of potential measured between the target and the retarding grid. Of course, the potential may be off the real energy by the difference in work functions of the stainless steel grids and the platinum target. All the KE distributions show a maximum near 0.3 eV relative KE with a tail at higher energy. The KE distributions at Ei = 9.5 eV are broader and have a higher maximum energy.
Discussion DEA in gas-phase methanol has been previously reported,15-17 but there are no data available in this phase about H-formation
via DD. The work of Kuhn et al." has focused on the release of ionic fragments whose masses are higher than that of H-. Curtis and Walker15 have observed three well-defined maxima in the D- ion ESD yield from CD30D and CH30D at incident electron energies of 6.5, 8, and 10.5 eV, with an onset near 5 eV. These three maxima are also reported in the early work by von Trepka and Neuert,16 but at slightly higher Ei values. As regards the first two DEA structures at 5-9 eV, the results of Curtis and Walkerls for partially deuterated methanols show the D- ions to come exclusively from the OD group, with dissociation of the 0 - D bond to D-('S) f CD30'(X2E). These authors15 also propose that these two lowest resonant states are Feshbach (one-hole,two-electron states) resonances. In contrast, for the third structure around 10.5 eV, the D- ions are shown to originate in both the hydroxyl and the methyl groups. The ESD of H- from CH30H multilayer films exhibits significant differences to the gas-phase results. The H- ESD yield function in the condensed phase (Figure 1) is considerably less structured than that in the gas phase, with an onset shifted up in energy by about 1 eV. Furthermore, inspection of Figures 2 and 3 suggests that the H- signal from condensed-phase methanol originates in both the hydroxyl and the methyl groups over all the considered electron energy range, while it exclusively comes from the OH group at 5-9 eV in gas-phase experiment^.'^ The origin of the DEA signal in the condensed phase must, however, be verified due to the potential occurrence of post-dissociation interactions of DEA anion fragments with surrounding molecules1*or anion-molecule isotope-exchange reactions.18 Taking CH30D as an example, such condensedphase effects may be written as
+ CH,OD - D- + CH30' D- + CH,OD - H- + CH,DOD e-
(1)
By using small amounts of solid methanol, it is possible to minimize these latter effects. The data shown in Figure 4 on the D- ESD yield functions measured from submonolayer(-0.1 ML) films of CH30D and CD30H on 20-ML-thick films of krypton are most important in this regard. Indeed, inspection of Figure 4 indicates that, for incident electron energies above 6 eV, the D- signal comes from both functional groups, which
10280 J. Phys. Chem., Vol. 98, No. 40, 1994 is a clear confirmation that in solid CH30H the H- DEA signal originates in both the hydroxyl and the methyl groups. By further comparing the results of Figures 2 and 3, we observe that the DEA H- and D- signals arising from the same functional group from both solid CH30D and CD30H are approximately identical for the hydroxyl group but slightly differ for the methyl group. In fact, the D- ions coming from the methyl group in Figure 3 have a stronger contribution in the 8-eV region. Incidentally, such a difference is also observed in gas-phase ESD experiments for the formation of CH3O- from CH30D compared with that of CD30- from CD30H.17 Although this difference cannot be quantitatively assessed with the present experiment, its origin can be seen through a variation in the relative contribution of the D- ions, in relation to the difference in the lifetime of the two lower-lying DEA resonances of CD30H- at 7.2 and 8.6 eV. By analogy with the ESD results from gas-phase methanol, the first two H- DEA resonance structures observed in multilayer films at 7.2 and 8.6 eV (Figures 2 and 3) or in submonolayer coverages on Kr substrates at 6.9 and 8.7 eV (Figure 4) probably arise from the same transient anion state as that found in the gas phase at 6.5 and 8 eV.15 The shift to slightly higher energies of these states upon condensation is expected from Feshbach resonances associated with Rydbergtype parent neutral states. Such anion states are composed of two Rydberg electrons temporarily bound to a positive molecular ion core.2 As known from spectroscopic data, the energy of a Rydberg orbital in the condensed phase is blue shifted from its gas-phase c~unterpart.'~ The present ESD results for condensed methanol can be compared with corresponding data obtained for amorphous solid water. The H- DEA desorption yield from multilayer H2O filmsz0has an onset at Ei = 5.5 eV and a single dominant peak located at 7.4 eV with a fwhm of about 2.4 eV. Above 8 eV, the anion yield function broadens, with an indication of a second smaller peak near 9 eV.*O These data resemble those observed in gas-phase Hz0,21,22the most apparent modification induced by condensation of HzO being a shift in the main peak of the H- DEA signal by 0.9 eV to higher incident electron energies. This shift has been associated with the modification of the excited-state potential energy surfaces caused by the perturbation of the 3s Rydberg orbital in the solid phase.20 As for water, the low-lying excited electronic states of methanol are all of Rydberg type,19,23so that the resonant states giving rise to DEA in condensed-phasemethanol are also expected to shift to higher incident energies (with respect to the gas phase). Because of the hygroscopic property of methanol, it may be difficult to avoid any water contamination. Experimentally, this may add some H- ESD signal in the 7.4-eV electron energy region, even though, in gas-phase methanol, some signal is also present at this energy. The effect of depositing 0.1 ML of methanol on 20 ML of solid Kr amounts to isolate the molecules of methanol on a thick rare-gas matrix so that inelastic multiple scattering of electrons by neighboring methanol molecules prior to DEA events is considerably reduced. Moreover, with the exception of multiple phonon losses, the absence of significant electron energy-loss processes in Kr below the threshold for electronic excitations (-10.2 eV in Kr multilayer films24)makes the ESD structures more apparent, as can be seen from the comparison between Figures 2 or 3 and Figure 4. The distinctive difference which is introduced by the Kr substrate is a new, narrow peak at 9.6 eV (Figure 4). This new D- ESD peak is observed from both CD30H and CH30D and can be ascribed to a two-step process, initiated by the resonant formation of anionic excitations in the
Parenteau et al. substrate rare-gas solid, and followed by a mechanism of simultaneous charge and energy transfer to the adsorbed target molecules.10 In fact, according to Rowntree et al.,1° Feshbachtype core-excited resonance states (or electron-exciton complexes) can be formed in the rare-gas substrate by the temporary binding of an incident electron to the lowest (Frenkel-type)bulk exciton via the polarization potential of this latter. Such states have recently been observed in Ar, Kr, and Xe multilayer films from high-resolution electron-energy-10~~~ and ESD9Joexperiments, where they are found to decay by releasing substantial amounts of energy to the lattice through multiphonon excitations or by transferring the excess charge and excitation energy to dissociative anion states of molecular absorbates, leading to enhanced desorption yields. These resonances are long-lived and,lie slightly below (-0.5 eV) the energy of the first electronically excited neutral state of the rare-gas substrate. Molecular orbital considerations1° indicate that the latter decay process is of observable magnitude only when electron exchange occurs between orbitals having a strong Rydberg character. The Rydberg nature of states involved in the mechanism of charge and excitation energy transfer from the core-excited resonance states of the Ar, Kr, and Xe substrates to dissociative anion states of adsorbed molecules has been c o n f i i e d for H20, C2D6, and c& physisorbed on the rare-gas solids.1° In the case under considerationhere, the transfer from the transient anionic exciton of solid krypton to the molecular target species CH30D or CD3OH, which must occur when the discrete energies of the initial and final states are equal, thus produces the sharp D- ESD peaks observed at 9.6 eV (Figure 4),characteristic of the substrate rare-gas resonance. lo Incidentally, this enhanced D- ion yield also confirms the Rydberg nature of the 8.6-eV dissociative anion state in pure condensed methanol. In view of the sensitivity of the charge and energy transfer to the nature of the Rydberg orbitals involved,10it is remarkable to find that the 9.6-eV peaks in the CH30D/Kr and CD30H/Kr solids have approximately the same magnitude with respect to the direct DEA process to both the methyl and the hydroxyl groups. This observation strongly suggests that both D- DEA decay channels arise from a common negative ion transitory state. As can also be seen from Figure 4, the two D- ion yield functions show a slight difference between the two partially deuterated methanols, depending on the origin site of the Dions. The D- ion yield from CH30D physisorbed on the Kr substrate shows two maxima at Ei = 6.9 and 8.7 eV that may be correlated with the 6.5- and 8.0-eV D- ESD peaks observed in the gas phase. In fact, the blue shift of the Rydberg states generally observed upon conden~ation'~ may again be invoked here to explain the shift to higher electron energies of these two gas-phase resonance states. The available KE of a dissociating anion fragment is an important parameter in DEA-induced ESD experiments. If the KE is not sufficiently high to overcome the induced polarization energy of the solid, the anion cannot escape into vacuum and therefore cannot be detected. Typical values of the polarization energy at the surface and in the bulk of solid Kr have been estimated to be about 1 f 0.3 eV.26 DEA data in gas-phase methanol indicate that the KE distributions of OH- and 0- ions, which appear out of the 10.5-eV resonance, have maxima at near-thermal energies and -1 eV, respectively." Hence, if similar dissociation mechanisms are also operative in condensedphase methanol, the OH- ions are not able to leave the surface and there is only a small probability for the emission of 0-, in agreement with the absence of these signals in our ESD experiments. Similar behavior has been observed by Stockbauer et al.27 in the ESD of positive ions from condensed CH30D
Electron-Stimulated Desorption of H- Ions and CD30H at 65-eV incident electron energy. The dominant ionic product is H+ (D+) with less than 2% of higher mass fragment ions. Such results differ from those obtained in the gas phase, where high-mass fragments are seen in ESD. Using isotopic substitution, these authors27have also demonstrated that the primary route to H+ formation in the energy range 30-65 eV is the excitation and decomposition of the methyl group, in contrast to the gas-phase results where H+ is shown to arise from a mixed process involving both CH3 and OH groups.28 Interestingly, our H- and D- signals generated via DD from both condensed CH30D and CD30H are more intense for the methyl group than for the hydroxyl one relative to the corresponding signals produced by the DEA process (see Figures 2 and 3). For DEA in gas-phase CD30D, the most probable KE of the D- ion distribution has a nearly linear variation as a function of the incident electron energy, with a slope of 0.72 over the 5-9-eV range of the two lowest resonances (Figure 6 of ref 15). The value of this slope is slightly lower than the maximum possible value of 0.94 expected from conservation of momentum and energy for the two fragmentation products D- and CD30.l5 According to Curtis and Walker,15 these results are consistent with a rapid 0-D bond rupture with, on average, about 75% of the total energy release appearing as translational energy of the two products. In the case of the third resonance near Ei = 10.5 eV, the most probable KE of the D- ions increases very slightly, with a slope of 0.18, the maximum value in the KE distribution being around 1.5 eV.15 This suggests a slow process with efficient redistribution of energy through the anion, prior to dissociation, which produces ions of relatively low energies and high masses. In this picture, the D- ions are supposed to leave from equivalent sites in a relatively long-lived, rearranged complex (possibly CD20Dz-).l5 Such a rearranged intermediate would explain the D- ESD signal coming from the methyl group around 10.5 eV. It is worth noting that the low KE distribution observed for the D- ions out of the third resonance will affect more the D- than the H- because of the difference in mass that gives a lower velocity for the desorbing D- ions. That may explain why, in the condensed phase, the D- signal from CD30H is less intense than that for H- from CH30D in the 10-eV region (see Figures 2 and 3). The KE distributions of the D- ions desorbing from solid CH30D and CD30H (Figures 5 and 6) show a maximum near 0 eV with a tail at higher energy, depending on the incident electron energy. The marked downward shift of the most probable KE from -3.5 eV in the gas phase (corresponding to the considered energies Ei = 8 and 9.5 eV)I5 to near 0 eV upon condensation is indicative of a large degree of inelastic anion scattering at or near the surface following the DEA event a n d or an enhanced probability of vibrational excitation of the neutral fragment during dissociation. The presence of inelastic processes prior to DEA may also affect the KE of the desorbing anions. In fact, the increased visibility of the two resonant structures in the D- ESD yield from CH30D in going from pure thick films to submonolayer coverages on Kr is a signature of the presence of inelastic collisions before DEA. We should note the remarkable similarity of the present results for the DKE distributions to the corresponding ones observed in solid amorphous water .2o Conclusion The results presented in this work demonstrate that both dissociative electron attachment and dipolar dissociation (above -12 eV) take place in solid methanol films. H- elimination is the only DEA channel that we have detected. The study of ESD on the isotopically labeled methanols CH30D and CD3-
J. Phys. Chem., Vol. 98, No. 40, 1994 10281 OH has allowed us to identify the originating sites of the Hions in condensed CH30H. It is found that, contrary to the gas phase, the H- ESD signal comes from both the hydroxyl and methyl groups in the electron energy range below 20 eV. Nevertheless, below 9 eV, the H- yield originates principally in the rupture of the 0-H bond with formation of CH30’ radicals. Above 9 eV, a substantial contribution to the desorption of H- ions also arises from C-H bond cleavage with production of eH20H radicals. Both of these dissociation channels have been found to arise from the same transient anion state near 10 eV. The release of H- ions from the hydroxyl group has been shown to involve dissociative Rydberg anion states, in close similarity with ESD data of H- formation in amorphous ice. It has also been shown that multiple electron scattering processes in the methanol films enlarge the width of the peaks in the yield functions and that post-dissociation interactions may be involved in the reduction of the kinetic energy of escaping anions. Acknowledgment. The authors thank Professor P. Rowntree for useful discussions. This work has been supported by the Medical Research Council of Canada. References and Notes (1) See, for example: Dixon-Warren, St. J.; Jensen, E. T.; Polanyi, J. C. J. Chem. Phys. 1993, 98, 5938. (2) See, for example: Electron-Molecule Interactions and Their Applications; Christophorou, L. G., Ed.; Academic Press: Orlando, 1984; Vols. 1 and 2. (3) For a review of hot-electron interactions in thin-film dielectrics, see: Sanche, L. IEEE Trans. Electr. Insul. 1993, 28, 789. (4) Vtzina, C.; Sanche, L. J. Chim. Phys. 1991, 88, 717. (5) For a review of the processes leading to negative-ion formation by electron impact in gases and their applications, see, for example: Massey, H. S. W. Negative Ions; Cambridge University Press: London, 1976. (6) For reviews of dissociative electron attachment processes in the solid phase, see, for example: Sanche, L. Comments At. Mol. Phys. 1991, 26, 321. In Desorption Induced by Electron Transitions, DIET V; Bums, A. R., Stechel, E. B., Jennison, D. R., Eds.; Springer Series in Surface Sciences, Vol. 31; Springer-Verlag: Berlin, 1993; p 3. (7) Sambe, H.; Ramaker, D. E.; Parenteau, L.; Sanche, L. Phys. Rev. Lett. 1987, 59, 236. (8) Azria, R.; Parenteau, L.; Sanche, L. J. Chem. Phys. 1987,87,2292. (9) Rowntree, P.; Parenteau, L.; Sanche, L. Chem. Phys. Lett. 1991, 182, 479. (10) Rowntree, P.; Sambe, H.; Parenteau, L.; Sanche, L. Phys. Rev. B 1993,47,4537. (1 1) Azria, R.; Parenteau, L.; Sanche, L. Phys. Rev. Lett. 1987,59,638. (12) Sanche, L.; Parenteau, L. Phys. Rev. Lett. 1987, 59, 136. (13) Huels, M. A.; Parenteau, L.; Sanche, L. Chem. Phys. Lett. 1993, 210, 340; J. Chem. Phys. 1994, 100, 3940. (14) Sanche, L. J . Chem. Phys. 1979, 71, 4860. (15) Curtis, M. G.; Walker, I. C. J . Chem. SOC., Faraday Trans. 1992, 88, 2805. (16) Trepka, L. v.; Neuert, H. Z. Naturforsch., Teil A 1963, 18, 1295. (17) Kuhn, A.; Fenzlaff, H.-P.; Illenberger, E. J . Chem. Phys. 1988, 88, 7453. (18) Azria, R.; Parenteau, L.; Sanche, L. Chem. Phys. Lett. 1990, 171, 229. (19) Robin, M. B. Higher Excited States of Polyatomic Molecules; Academic Press: New York, 1974 and 1975; Vols. I and II. (20) Rowntree, P.; Parenteau, L.; Sanche, L. J . Chem. Phys. 1991, 94, 8570. (21) Melton, C. E. J. Chem. Phys. 1972, 57, 4218. (22) Belit, D. S.; Landau, M.; Hall, R. I. J. Phys. B 1981, 14, 175. (23) Wadt, W. R.; Goddard III, W. A. Chem. Phys. 1976, 18, 1. (24) Zimmerer, G. In Excited-State Spectroscopy in Solids, Proceedings of the International School of Physics “Enrico Fermi”, Varenna, Italy, July 1985, Course XCVI; Grassano, U. M., Terzi, N., Eds.; North-Holland: Amsterdam, 1987; p 37. Steinberger, I. T.; Bass, A. D.; Shechter, R.; Sanche, L. Phys. Rev. B 1993, 48, 8290. (25) Michaud, M.; Cloutier, P.; Sanche, L. Phys. Rev. B 1993,48, 11336. (26) Michaud, M.; Sanche, L. J. Electron Spectrosc. Relat. Phenom. 1990, 51, 237. (27) Stockbauer, R.; Bertel, E.; Madey, T. E. J. Chem. Phys. 1982, 76, 5639. (28) Burrows, M. D.; Ryan, S. R.; Lamb, W. E., Jr.; McIntyre, L. C., Jr. J. Chem. Phys. 1979, 71, 4931.
