J. Phys. Chem. 1995, 99, 6123-6127
6123
Absolute Cross Section for Dissociative Electron Attachment to CF4 Condensed onto Multilayer Krypton Andrew D. Bass,* Jerry Gamache, Luc Parenteau, and Leon Sanche Groupe du Conseil de Recherches Mtdicales en Sciences des Radiations, Facultt de Mtdecine, Universitt de Sherbrooke, Sherbrooke, Qutbec, Canada J I H 5" Received: October 24, 1994@
We present data showing the energy dependence of the electron-stimulated desorption signal and chargetrapping cross section for molecules of CF4 condensed onto a multilayer Kr film. In both cases, these phenomena occur via dissociative electron attachment (DEA) to the molecule. As with pure CF4 films, the only detectable desorbed signal from the film is F-, generated via the lowest of two CF4- resonant states associated with DEA. However, both resonant states are implicated in charge trapping via the formation of fragment ions at the surface. The charge-trapping cross section at maximum, is 4-7 times greater than the cross section for DEA in the gas phase. The enhancement in the DEA cross section is estimated to be between a factor 5 and 9. This enhancement reflects an increased probability of survival against autodetachment for intermediate anion states that results from interaction with the electronic polarization field they induce on the Kr surface.
Introduction It has been found that many of the phenomena observed following low-energy electron impact on condensed atoms and molecules' are related to elementary processes already known from the gas phase.2 In particular, at the lowest impact energies (< 10 eV) anion desorption is the result of dissociative electron attachment (DEA). Within this process, a molecular target M captures an electron via a resonance mechanism to form a temporary negative ion M(*)-. Subsequently,this anion decomposes to form a negative and a neutral fragment,
The first observation of DEA in condensed systems was via the electron stimulated desorption (ESD) of 0- from a multilayer 0 2 film at approximately 6 eV impact energy.3 This and subsequent studies have shown that in the condensed phase DEA is largely affected by the nature of surrounding molecules4and, depending on the thickness of the condensed layer, by the proximity of the metallic sub~trate.~These perturbations can affect the energy and lifetime of the temporary negative ion (M(*)-), the energy of the dissociation limits, and the dynamics of the overall process. Until now, desorption studies have provided a thorough qualitative understanding of the importance of these factors; however, a fundamental understanding of the interactions involved requires quantitative information, Le. absolute values of the cross sections for DEA in the solid phase. Such information is not easily obtained from desorption studies, and absolute cross sections cannot be measured since in these measurements most of the ions generated by DEA do not desorb, but remain trapped in or on the film. Marsolais et alS6have shown that, for a dielectric film surface doped with a molecule capable of trapping electrons and bombarded by electrons of well-defined energy, the rate of charging of the film is proportional to the absolute cross section for charge trapping (CT) at low doses. To date, the only measurement of @
Abstract published in Advance ACS Abstracts, April 1, 1995.
0022-365419512099-6123$09.0010
an absolute DEA cross section for a molecule condensed on a solid surface has been reported for submonolayer 0 2 physisorbed on a Kr multilayer film.7 The DEA cross section per H20 molecule within surface clusters has also been measured.8 In this paper we report on the anion desorption yield from CF4 molecules isolated onto a Kr surface and the CT by such a system. By comparing these results, we can deduce that CT is due to DEA and thus provide an absolute value for the cross section of this process. Tetrafluoromethane is a material of industrial and scientific relevance. Its use in plasma etching treatments and in glow discharges is well-kn~wn,~ and it is implicated, together with other halogen-containing hydrocarbons, in the destruction of stratospheric ozone.1° It is likely that ozone breakdown is a surface-mediated process occurring on the surface of ice crystals or dust particles. For this reason, absolute cross sections for processes occumng on surfaces and capable of producing reactive species (here F- and F) can be of considerable value.
Experimental Section The measurements for charge trapping and for ESD from CF4 were performed in two separate apparatus. The general arrangement of the instrument used to measure ion desorption has been described elsewhere." Briefly, it consists of an electron monochromator, a cryogenically cooled target, and a quadrupole mass spectrometer. These are housed within a conventional ultrahigh-vacuum (UHV) chamber maintained at a base pressure of mbar. The electrostatic monochromator produces an electron beam of 2 x A current, with a resolution of approximately 80 meV (fwhm). The beam strikes the target surface at an angle of 70' t o the normal. The electron energy scale is calibrated within &300 meV with respect to the vacuum level by measuring the onset of the transmitted current in the presence of the film. Samples were prepared by first condensing a 15-layer Kr film on a clean polycrystalline platinum foil and then depositing approximately 0.15 of a monolayer of CF4 onto its surface. The Pt substrate was maintained at 18 K, and the film thicknesses were estimated with an accuracy of 50% using a previously described gasexpansion method.12 Desorbed ions were collected with 0 1995 American Chemical Society
6124 J. Phys. Chem., Vol. 99, No. 16, 1995
Bass et al.
electrostatic lenses, mass selected, and detected by a channeltron electron multiplier as a function of the incident electron energy. A complete description of the apparatus for measuring chargetrapping cross sections, along with the theoretical background of this technique, has been given elsewheree6 However, the technique is derived from low-energy electron transmission (LEET) spectroscopy,12 which measures the electron current passing through a dielectric film and arriving at a metal substrate as a function of the potential applied between the substrate and the electron source. When the dielectric film is not charged and the electrons have just enough energy to enter the film, a sharp rise termed the “injection curve” (IC) is seen in the LEET spectrum. When electrons are trapped on the dielectric film surface, the IC shifts to a higher accelerating voltage because the trapped negative charges retard the incoming electrons. The shift in IC, AV, is related to the total surface charge Q by
1
I
1
1
I
0
a
F
?
E
I
I
1
1
I
I
6
8
10
m 0
~6
QL AV= €72
where r is an average radius of the surface charge spreading, and E and L are the dielectric constant and thickness of the film, respectively. When a dielectric film is partially covered by molecules with a small surface density 00 and only these molecules can trap electrons for a time t, the total surface charge Q(t) trapped by the molecules is given by
Q(t)
Io,$
(3)
for small t, where Z is the total electron current, and p is the total electron-trapping cross section. The corresponding shift AV(t) is given by AV(t) = tUo&nr2
(4)
and its time derivative at t = 0, which is denoted by A, is given by
A,
dAV(t)/dtl,,
= Ho,,u/arr2
(5)
The CT experiments were also performed under UHV conditions in a chamber ion pumped to a base pressure of mbar. Essentially, the apparatus consists of a trochoidal electron monochromator (electron beam resolution = 40 meV at 1 nA beam current) and a cryogenically cooled polycrystalline pt target, onto which the films are deposited. The monochromator is used to both charge the film and measure the injection curve. In these experiments 0.1 monolayer (&30%) of CF4 was deposited onto 15-monolayer-thick films of Kr held at a temperature of approximately 20 K. Film thicknesses were estimated to 15% accuracy by monitoring the formation of quantum-size interference structures in the LEET spectra of thin films as described p r e v i ~ u s l y . ~In~ both the CT and ESD measurements, the stated purities of Kr and CF4 were 99.995% and 99.95%, respectively. The electron beam radius was measured to be (3.7 f 0.3) x m.
Results and Discussion The new measurements are shown in Figure 1 together .with comparable results from gas phase experiments.14 Figure l a shows how the CT cross section changes with incident electron energy, while Figure l b shows the yield of F- desorbed from the film surface. The yields of F- and CF3- ions from gas phase electron-molecule collisions are shown in Figure 1, parts c and d, respectively. In the gas phase, the relative intensity of
4
Electron Energy (eV) Figure 1. (a) Charge-trapping cross section for 0.1 ML of CF4 on 15 ML of Kr. (b) F- yield from 0.15 ML of CF4 on 15 ML of Kr. (c and d) Yield of CF3- and F-,respectively, from gas phase CF4 (from ref 13). The absolute value of charge-trapping cross sections is accurate to *20%.
the two fragment ions is F-:CF3- =1:0.6.15 It is apparent from Figure 1 that there exist similarities among these four curves and that the ESD and CT phenomena observed in the condensed phase derive from essentially the same DEA process seen in the gas phase. Close inspection of the gas phase data does reveal some difference. While CF3- formation is associated with a Gaussian-like profile around 6.8 eV, the F- spectrum peaks at 7.6 eV and has a structure near 6.8 eV. This behavior has been interpreted as electron capture via two negative ion states, ground state CF4- and an electronically excited CF4*state.l4,l6 The electronic ground state decomposes along repulsive energy surfaces via the complementary channels CF4- (6.8 eV) CF,- (6.8 eV)
-
+ CF3F+ + CF3 F
(64 (6b)
while the excited state yields exclusively F-. Consequently, the F- yield is composed of contributions from these two overlapping resonances. In addition to producing F-, it is thought probable that CF4*- dissociates to form an excited CF3* radical that subsequently dissociates:16 CF4*- (7.6 eV)
-
F-
+ F + CF2
(7)
Such a picture is supported by an analysis of the translational energy r e l e a ~ ein~ the ~ , ~corresponding ~ processes. Both reactions 6a and 6b yield ionic fragments of high kinetic energy. The F- ions, produced from dissociation process 7, are of considerably lower kinetic energy, indicating a less direct decomposition mechanism involving electronic and vibrational predissociation. The ESD of ions from multilayer films of pure CF4 has been reported by Meinke et a1.l’ and is very similar to the data presented here for CF4 on multilayer Kr. In both cases the only
Dissociative Electron Attachment to CF4
J. Phys. Chem., Vol. 99, No. 16, 1995 6125 The value obtained here for the maximum charge-trapping cross section is 7.3 x cm2 f 37% and is significantly larger than the gas phase cross section by a factor of between 4 and 7. In fact, the enhancement in the condensed phase DEA cross section is somewhat greater than this, as some fraction of the F- ions desorb from the film. A direct measurement of the proportion of ions desorbing from the film is not possible with the present apparatus; however, it is possible to estimate this quantity and so obtain an approximate value for the DEA cross section in this system. For an ion to desorb from the film it must possess sufficient kinetic energy E k to overcome the surface polarization potential Ep. Moreover, as discussed by Huels et al.,*O the component of its momentum normal to the film's surface must be greater than some critical value p c given by
2000
1 2
i 4
6
8
10
12
14
Electron Energy (eV) Figure 2. Comparison of F- signal desorbed from (a) multilayer CF4 and (b) 0.15 ML of CF4 on 15 ML of Kr.
detectable ionic signal belongs to F-, and the yield peaks at 7 eV incident electron energy. A comparison of the F- signal seen in the present experiment and that seen in the work of Meinke et al. is shown in Figure 2. Note that the signal from the 15% CF4 covered Kr surface is approximately one-third as strong as that from the pure CF4 film. While superficially similar to the gas phase ion yield, an analysis of the kinetic energy of the desorbed F- ions in ref 17 showed that all ions originated from DEA via the lower energy resonant process (mechanism 6a). To desorb from the surface, ions must have sufficient kinetic energy to overcome the polarization energy of the surface. It appears that the low translational energy imparted to F- ions generated via mechanism 7 is insufficient for desorption. Similarly, CF3- is unable to leave the pure film surface despite the large excess energy available in the dissociation process in which it is formed. This is because only 22%of the available energy is imparted to this heavy fragment. Finally, it was suggested that the larger width of the condensed phase resonance might be the result of inelastic scattering prior to the formation of CF4- . Careful study of Figure 2 supports this hypothesis, as the ESD signal from the pure CF4 is noticeably broader, on the high-energy side, than that from the CF4-doped Kr film. There are obviously fewer inelastic processes capable of broadening the resonance in a Kr film. Because of the strong similarities between the data of Meinke et al. and the ESD results for CFq on Kr, we conclude that the same arguments apply in the present case. In contrast to the ESD data, the charge-trapping cross section reveals two structures, indicated in Figure l a by the two arrows: a maximum at 5.8 eV and a shoulder at 6.6 eV. This suggests strongly that at the surface of the film both resonance mechanisms 6 and 7 are involved in the charge-trapping process. Indeed, a shift of 1 eV to lower energy of the gas phase anion states places the two resonances at the energy of the two structures seen in Figure la. This energy shift compares well with the polarization energy 0.72 eV for the Kr surface.18 The 0.3 eV difference may be attributed to the fact that parent neutral Rydberg states are usually blue shifted upon condensation.11 The gas phase cross sections for the production of individual anionic fragments15 and for total DEAI9 have been measured and peak values found in the range (1.0-1.6) x cm2.
EP =
Pc
2
where m is the mass of the ion. The ion's momentum normal to the film's surface is
p(Z) = (2mEk)"2 cos Cp
(9)
where q5 is the desorption angle measured from the normal n. Consequently, to leave the film permanently, the following condition must be met:
Ek > Edcos Cp
(10)
For each value of E k there exists a maximum desorption angle Qmaxbeyond which escape from the film is impossible. In the gas phase, the average kinetic energy of F- ions created via DEA from the lower energy resonance at its maximum at 7 eV incident electron energy is 1.5 eV.14 With a surface polarization energy for multilayer Kr of 0.72 & 0.2 eV,20 is calculated to be 46.2'. Let us imagine the tetrahedral molecule CF4 physisorbed onto the Kr film. It is reasonable to assume that it sits with three fluorine atoms resting on the surface. The fourth F atom is thus positioned such that its C-F bond axis is normal to the film surface. Should DEA to the molecule result in rupture of this bond and the creation of an F- ion of 1.5 eV kinetic energy, the ion will desorb. If it is assumed that rupture is equally likely along each of the four C-F bonds, then 25% of the F- ions created via DEA will desorb immediately in vacuum. Rupture along the other three C-F bonds will inject a F- ion of 1.5 eV into the Kr film at an angle of approximately 109.5' to the surface normal. What fraction of these ions subsequently desorb? Ions entering the film are scattered by the lattice atoms. If they are scattered through a suitably large angle, they may reemerge from the film surface. The initial kinetic energies of the ions are significantly larger than the excitation energies of the lattice phonon modes, and the majority of ions undergo elastic scattering close to the surface. Huels et aLZOargue that because binding energies within the lattice are small compared to the initial kinetic energy of ionic fragments, an elastic scattering event within the lattice can be approximated to an isolated binary collision between an ion and a Kr atom and can be adequately described by classical mechanics. With masses of 19 and 84 a m u for F- and Kr, respectively, the ion can give part of its kinetic energy to the Kr atom. The larger the scattering angle, the lower the residual energy of the ionic fragment. Our calculations show that for ions of initial kinetic energy less than 1.3 eV and injected into the film at an angle of 109.5' to the surface normal no ions undergoing a single
6126 J. Phys. Chem., Vol. 99, No. 16, 1995
Bass et al.
8
b) CF,/Kr r
6
$ 4 v
h
P a
C w 2
0
Re
R,
Re R,’ Distance C-F (arb units)
Distance C-F (arb units)
Figure 3. Schematic diagram showing the lowest potential curves for CF4 and CF4- (a) in the gas phase and (b) on multilayer Kr.
collision can desorb from the film. At an initial kinetic energy of 1.5 eV only those ions scattered within a narrow cone of half-angle 18” can escape the film. If we assume isotropic scattering, approximately 3% of ions injected into the film can desorb. In fact, the classical isolated binary collision is not an isotropic process, and small angle scattering, corresponding to the ion striking the Kr atom at glancing incidence, is more probable than large angle scattering. However, the periodic nature of the lattice reduces the number of glancing collisions and so enhances the probability of large angle scattering. Bearing these points in mind, and considering the possibility of desorption following multiple collisions, we estimate that between 5% and 10% of the ions entering the film can desorb. Consequently, we estimate the total fraction of ions leaving the film to be (30 & 3)%. At incident electron energies close to 7 eV the F ions correspond to approximately 50% of the total DEA ion yield; all the CF3- ions remain on the Kr film, so roughly 15% of the total number of ions produced leave the surface. This implies that the cross section for DEA is approximately 20% larger than that for CT. If we assume that the maximum value of the CT cross section corresponds to the production of 1.5 eV F- ions, we estimate the maximum DEA cross section to be 9 x cm2 & 50%. Thus, with this correction for anion ESD in vacuum, the cross sections in Figure l a are 5-9 times larger than the corresponding ones in the gas phase. Significant enhancements in the DEA cross sections of isolated molecules on the surface of rare gas solids have been seen previously. An enhancement of factor 20 has been measured for 0 2 molecules on a Kr film.’ In that case, the enhancement was attributed to an increase in survival probability against autodetachment by the intermediate 0 2 - state. This was caused by the electronic polarization of the Kr surface that shifted the potential energy curves of anion states relative to those of neutral states, thus altering transition rates between these two species. Figure 3 shows in schematic form the lowest states of CF4 and CF4- and the effect of the local polarization field. When a CF4 molecule is placed on the Kr film, as shown in Figure 3b, the potential curve for the neutral ground state remains virtually unchanged, but the curves of the anion state are shifted downward due to the polarization field of Kr
neighbors. This effect is apparent in the charge-trapping data which show that the position of the resonance maxima shifted approximately 1.0 eV below the gas phase values and ESD results.21 A further effect is that the dissociation limits of the anion, FCF3, and F CF3- are each split by the surface polarization energy (0.72 eV) into two limits, one corresponding to the desorption of the fragment ion from the surface (CF3/Kr F- and F/Kr and CF3-, respectively) and one corresponding to the trapping of the ion at the surface (F-/Kr (CF3 or CF3/ Kr) and C F 3 - k (F or F/Kr), respectively). Note that the dissociation limits for ion desorption are approximately the same as the gas phase limits, while the limits for ion trapping are lower by an amount equal to the polarization energy. Also note that the position of these limits is unaffected by whether the neutral fragments remain on or desorb from the film. In this picture, DEA corresponds to the capture of an electron via a vertical Franck-Condon type transition from the electronic ground state onto the CF4- potential curve, where the molecule/ electron system remains long enough to relax and dissociate. Therefore, the effective DEA cross section (and that for charge trapping via DEA) depends strongly on the probability of survival P(R) for the CF4- ion against autodetachment and its retum to the electronic ground state. O’Malley22 gives an expression for this survival probability:
+
+
+
+
+
with R being the distance along the reaction coordinate at the time of electron capture (here the distance between the carbon and a fluorine atom). P(R) depends on the electron energy (starting point, to(R));the steepness of the negative-ion potential curve (velocity, tl(Rc));the autoionization width of the negativeion potential curve (leakage, Ta(R));and the position of the curve crossing point R,, beyond which autoionization is no longer possible (see Figure 3). Lowering the negative-ion potential curve by placing CF4 on the Kr surface has the effect of bringing R, closer to the original equilibrium position of the electronic ground state Re and consequently increases the survival probability against autodetachment for ions created at Re. This effect is shown in Figure 3 for a lowering by 1 eV of the CF4- ground
J. Phys. Chem., Vol. 99, No. 16, 1995 6127
Dissociative Electron Attachment to CFa state potential energy curve. We propose that this mechanism is responsible for the observed increase in the cross section for DEA to condensed CF4 withrespect to the gas phase process. In Figure 3 we have assumed that the potential energy curve of the anion is shifted down uniformly along the C-F bond axis, by the surface polarization energy. However, the arguments advanced to describe the enhancement in DEA do not require a uniform shift. Any downward shift of the anionic curve will bring R, closer to Re and so increase the proportion of anions that dissociate. We believe that a “near uniform” shift is likely, given that the surface polarization energy induced by a localized charge is dependent only on the distance of the charge from the film-vacuum interface.’* This distance is likely to change during dissociation but only slightly, and this effect can be considered as a perturbation of the surface polarization energy. A further perturbation might be the extent to which F- and CF4- can be considered as localized charges. However, we note that in the present case, a grossly nonuniform shift would cause a significant change in the width of the DEA cross section relative to the gas phase. No such effect was observed. For the case of 0 2 on Kr,’ detailed knowledge of the potential energy curves of the neutral molecule and the resonance anion allowed an estimate of the enhancement in DEA cross section to be calculated. Excellent agreement was found with the results of the charge-trapping experiment. Unfortunately, the potential energy surfaces of CF4 and CF4- are not available for a similar calculation to be performed for this system. We note that autodetachment from the 0 2 - ion predominately produces 0 2 * excited neutral states lying immediately below the anion. The threshold for electronic excitation in CF4 is at 12.4 eV;23 therefore any autodetachment must be to the electronic ground state and its vibrationally excited levels. Indeed, Mann and LinderZ4have shown in the gas phase that the same resonant states associated with DEA greatly enhance the cross section for vibrational excitation. An estimate of the total resonant excitation cross section for all vibrations is given as 5 x cm2, more than 2 orders of magnitude greater than the gas phase cross section for DEA. Thus, despite the enhanced DEA cross section in the condensed phase, autodetachment to the electronic ground state must remain the predominant outcome of resonant electron capture by CF4 on the Kr surface, unless the initial electron capture probability is much lower on the surface than in the gas phase.
Conclusion The present results show that, as in the gas phase, low-energy electron impact on CF4 molecules condensed onto a Kr surface produces anionic and neutral fragments via DEA. A comparison of CT and ESD data shows that the two resonant CF4- states, previously identified in gas phase measurements, contribute to DEA in the condensed phase. The CT method described in this paper allows an absolute cross section for anion stabilization at the surface to be measured and an absolute cross section for total DEA to be estimated. A significant enhancement in cross section was seen and explained in terms of a preexisting model in which the potential energy curves of the intermediate CF4ion are displaced by the polarization energy of the Kr surface.
This displacement reduces the distance through which a CF4ion must relax before autoionization to the neutral molecule becomes impossible and increases the proportion of ions that dissociate. The extent to which a surface can enhance a DEA cross section is significant in understanding DEA’s role in the reactions of absorbed molecules under photon or electron b ~ m b a r d m e n t . ~That ~ . ~ such ~ surface-mediated reactions may be responsible for the observed seasonal reduction of atmospheric ozone is of particular relevance and suggests that the present measurements should be extended to other halogencontaining hydrocarbons absorbed onto a variety of different substrata. Such work is presently underway.
Acknowledgment. This work has been supported by the Medical Research Council of Canada. We thank Dr. M. Huels for several illuminating discussions. References and Notes (1) Sanche, L. Excess Electrons in Dielectric Media; Ferradini, C., JayGerrin, J.-P., Eds.; CRC: Bocca Raton, 1991; Chapter 1, Primary Interactions of Low Energy Electrons in Condensed Matter. (2) Christophorou, L. G. Electron-molecule interactions and their applications; Academic Press: Orlando, 1984; Vols. 1 and 2. (3) Sanche, L. Phys. Rev. Len. 1984, 53, 1638. (4) Huels, M. A.; Parenteau, L.; Sanche, L. J. Chem. Phys. 1994, 100, 3940. (5) Sanche, L. Comments At. Mol. Phys. 1991,26, 321 and references therein. (6) Marsolais, R. M.; Deschbnes, M.; Sanche, L. Rev. Sci. Instrum. 1989, 60, 2724. (7) Sambe, H.; Ramaker, D. E.; Deschbnes, M.; Bass, A. D.; Sanche, L. Phys. Rev. Lett. 1990, 64, 523. (8) Bass, A. D.; Sanche, L. J. Chem. Phys. 1991, 95, 2910. (9) Melliar-Smith, C. M.; Mogab, C. J. Thin Film Processes; Vossen, J. L., Kem, W., Eds.; Academic: New York, 1978; p 495. (10) Garcia, R. Physics World 1994, April 7, 49. (11) Rowntree, P.; Parenteau, L.; Sanche, L. J . Chem. Phys. 1991, 94, 8570. (12) Sanche, L. J. Chem. Phys. 1981, 71, 4860. (13) Perluzzo, G.; Sanche, L.; Gaubert, C.; Baudoing, R. Phys. Rev. B. 1984, 30, 4292. (14) Illenberger, E. Chem. Phys. Lett. 1981, 80, 153. (15) Garland, P. W.; Franklin, J. L. J. Chem. Phys. 1974, 61, 1621. (16) Oster, T.; Kuhn, A.; Illenberger, E. Int J. Mass Spectrom. Ion Processes 1989, 89, 1. (17) Meinke, M.; Parenteau, L.; Rowntree, P.; Sanche, L.; Illenberger, E. Chem. Phys. Len. 1993, 205, 213. (18) Michaud, M.; Sanche, L. J. Electron Spectrosc. Relat. Phenom. 1990, 51, 237. (19) Hunter, S. R.; Christophorou, L. G. J. Chem. Phys. 1984,80, 6150. (20) Huels, M. A.; Parenteau, L.; Michaud, M.; Sanche, L. Phys Rev A , in press. (21) The energy difference between the CT and ESD data is possibly related to the escape probability of ions of different kinetic energies. The lower the incident electron energy, the lower the kinetic energy of F- ions created via DEA and the lower the probability of escape from the film. Such an effect produces an apparent energy difference between maxima in the ESD and CT spectra. (22) O’Malley, T. F. Phys. Rev. 1966, 150, 14. (23) Robin, M. B. Higher Excited States of Polyatomic Molecules; Academic: New York, 1974; Vol. 1. (24) Mann, A.; Linder, F. J. Phys B: Afmos. Mol. Opt. Phys. 1992,25, 545. (25) Zhou, X.-L.; Zhu, X.-Y.; White, J. M. Surf: Sci. Rep. 1991,13, 73. (26) Sanche, L. Desorption Induced by Electron Transistions DIET V; Burns, A. R., Stechel, E. B., Jennison, D. R., Eds.; Springer-Verlag: Berlin, Heidelberg, 1993; p 3. Jp9428653
