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J. Phys. Chem. B 2000, 104, 4711-4716

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Electron-Stimulated Desorption of H- from Condensed-Phase Deoxyribose Analogues: Dissociative Electron Attachment versus Resonance Decay into Dipolar Dissociation D. Antic, L. Parenteau, and L. Sanche* Groupe du Conseil de Recherches Me´ dicales du Canada en Sciences des Radiations, Faculte´ de Me´ decine, UniVersite´ de Sherbrooke, Que´ bec J1H 5N4, Canada ReceiVed: January 14, 2000; In Final Form: March 8, 2000

We report the 5-40 eV electron-stimulated desorption (ESD) yields of H- from thin films of the DNA backbone sugarlike analogues tetrahydrofuran (I), 3-hydroxytetrahydrofuran (II), and R-tetrahydrofurfuryl alcohol (III), as well as ESD yields from submonolayer amounts of these compounds condensed on multilayer Ar films. For the pure disordered solid films, our results corroborate the previous observation of a peak in the H- yield function at an incident electron energy, Ei, of ∼10 eV attributed to dissociative electron attachment (DEA) along the CH bonds via the formation of a core-excited resonance. For II and III, a second lowenergy peak is also observed in the H- ESD yield function. This peak appears near an Ei of 7.3 eV as a weak shoulder superimposed on the low-energy side of the 10 eV structure; it is associated with a core-excited Feshbach resonance leading to H- production via DEA to the OH substituent. For each of the three molecules, we observe near 23 eV a third broad peak in the H- ESD yield functions. Measurements of the H- yields, as a function of the coverage of thin Ar spacer films by I, II, and III, suggest that the 23-eV peak is not due to multiple electron scattering, but results from direct transient anion formation above the dipolar dissociation threshold. These results, combined with those recorded at higher Ar spacer thickness, indicate that the 23-eV peak arises principally from decay of a transient anion (or anions) into an electronic excited state (or states), which dissociates into H- and the corresponding positive ion radical.

I. Introduction Low-energy electron (E < 20 eV) damage to condensed molecules has been the focus of intense studies in recent years.2 At these low energies, ion production by electron impact results essentially from two processes: namely, molecular fragmentation into a stable anion and neutral species, which occurs via dissociative electron attachment (DEA), and fragmentation into an ion pair, that is, dipolar dissociation (DD).1,2 Conceptually, DEA in molecular solids is similar to that found in the gas phase; an electron impinging on the solid is temporarily captured in a virtual orbital of one of the molecular constituents to form a transient negative ion state (NIS) near or at the surface. If the NIS is relatively long-lived (i.e., >10-14 s) and dissociative within the Franck-Condon (F-C) region of the neutral groundstate target molecule, dissociation into a stable anion and groundor excited-state neutral fragments may occur. DEA typically involves core-excited states (two-electron, one-hole), where the excess electron is bound to an electronically excited molecule. For a molecular target R-H, the DEA process may be represented by the reaction

e- + [R-H] f [R-H]- f R• + Hif the state [R-H]- is dissociative along the R-H coordinate. The residual energy from the reaction is shared as kinetic energy (KE) of the fragments R• and H-, plus the internal energy of the radical R•. This latter is composed of intramolecular vibrations with or without electronic excitation depending on the dissociation limit. Partitioning of the excess energy depends largely on the initial anion configuration, the F-C factors, and * Corresponding author.

the available degrees of freedom. The intensity of the H- anion yields strongly depends on their initial KE, because only those that overcome the induced surface polarization potential can escape the surface. Electron-stimulated desorption (ESD) of stable anions may also arise from DD, which involves dissociation of an electronically excited molecule into an ion pair. A priori, DD may be resonant or direct. For the molecule R-H, these processes may be represented as

e + [R-H] f [R-H]- f [R-H]* + e f R+ + H- + e in the case of the resonant process and as

e + [R-H] f [R-H]* + e f R+ + H- + e for direct DD. The DEA and DD processes are routinely monitored in ESD experiments, which relate the negative ion yield desorbing from the surface of a multilayer film to the incident electron energy. Each resonance is characterized in the ESD anion yield function by a broad and, typically, featureless peak. Ordinarily, lowenergy ESD studies are concentrated below about 15 eV, that is at energies where DD does not contribute significantly to the stable anion signal. Above the DD threshold, the anion ESD yield is usually dominated by anions produced via pair production, but resonance-like structures are occasionally observed above this threshold. In this case, broad peaks are superimposed on the direct DD anion yield contribution, which increases monotonically with electron energy from threshold. Maxima or resonance-like structures appearing in the ESD anion yield above the DD threshold have been shown repeatedly3-5 to be the result

10.1021/jp000206m CCC: $19.00 © 2000 American Chemical Society Published on Web 04/20/2000

4712 J. Phys. Chem. B, Vol. 104, No. 19, 2000

Figure 1. Chemical identity of the DNA backbone sugarlike analogues tetrahydrofuran (I), 3-hydroxytetrahydrofuran (II), and R-tetrahydrofurfuryl alcohol (III).

of multiple electron scattering (MES) within the bulk solid before electron attachment. In the case of amorphous ice, however, the presence of a core-excited resonance has been proposed6 as being partly responsible for the anion yield at energies above the DD threshold. In the DD process, one of the charged products remains on the surface, providing additional energy from polarization to the receding ion,3,4 which thus has a larger probability to escape the induced polarization potential. In contrast, when anion desorption occurs via DEA, this additional energy is not available because no ion stays at the surface. When all forces are taken into account, the net effect is seen to produce an increase in DD yields close to a metal substrate, due to the larger charge-induced polarization,4 whereas DEA is reduced close to a metal surface.3 As we show in this paper, it thus becomes possible to identify contributions from DEA and DD by modifying the distances of anion formation from a metal substrate while monitoring changes in the magnitude of the different peaks in anion ESD yield functions. Generally, the energies required for heterolytic bond dissociation are far greater than those needed for DEA, with threshold energies for ionpair formation typically near 12-15 eV. Low-energy ESD has particular relevance to the field of radiation science, as it is well known that large amounts of lowenergy secondary electrons, with initial KE well below 30 eV, are produced along radiation tracks following the interaction of ionizing radiation.1 Given the propensity for electron damage at these energies, it is not surprising that in recent years there has been a shift in ESD studies toward molecular systems of biological relevance, such as DNA bases,7 radiosensitizing 5-halouracils and homo-oligonucleotides.8 We are now studying the chemical consequences of the interaction of low-energy electrons with the deoxyribose backbone of DNA and its constituents. In a recent paper (hereafter referred to as A), we reported on ESD and high-resolution electron energy loss (HREEL) studies on DEA-induced damage to the DNA sugarlike backbone analogues tetrahydrofuran (I), 3-hydroxytetrahydrofuran (II), and R-tetrahydrofurfuryl alcohol (III) condensed at 27 K on polycrystalline Pt.9 In each model compound we found only H- desorbing from the surface, but HREEL measurements suggested that the dissociation channels in at least tetrahydrofuran might be more complex. For all three compounds, a broad peak at 10 eV in the H- ESD yield function was found to arise from selective dissociation of the RC-H bonds. A second lower-energy resonance near 7.3 eV in II correlated well with a Feshbach resonance observed in solid methanol,10,11 though a similar Feshbach resonance in the HESD yield from III was not apparent. In this paper, we study the coverage dependence of ESD yields from compounds I, II, and III, whose nomenclature is shown in Figure 1. We also examine in greater detail the 5-15eV region and extend our earlier work on I, II, and III to the 15-40-eV range. We corroborate previous results and find that the 7.3-eV resonance is also present in III. At higher energies,

Antic et al. a broad peak is observed near 23 eV in the H- ESD function. From the analysis of the film thickness dependence of the magnitude of the 10- and 23-eV peaks, and that of the 40-eV signal, we differentiate contributions from DEA, MES, and direct or resonant DD. The major portion of the 23-eV signal is found to arise from electron attachment followed by decay of the resonance into the DD channel. To our knowledge, this is the first evidence of ESD via resonance decay into DD. II. Experimental Methods The apparatus operating modes and experimental method have been described in detail elsewhere.12-15 Briefly, multilayer films of I, II, and III and Ar atoms are grown in a vacuum on an electrically isolated polycrystalline Pt ribbon attached to the tip of a closed-cycle helium refrigerated cryostat. The Pt ribbon is cleaned by resistive heating to ∼800 °C. A collimated 4-5 nA incident electron beam emanating from a hemispherical monochromator with a full width at half-maximum of ∼80 meV impinges onto the target film at an incident angle of 70° from the surface normal. All components are housed in an ultrahigh vacuum system reaching a base pressure of ∼2 × 10-10 Torr. The incident electron beam energy Ei is calibrated to within (0.15 eV of the vacuum level by measuring the onset of current transmission through the multilayer film.16 A fraction of the negative ion flux desorbing from the surface at an angle of 20° at the opposite azimuth is focused into a quadrupole mass spectrometer by electrostatic lenses. Charging of the film during the experiments is monitored by measuring changes in the energy onset of the transmitted current. When charge accumulation results in a shift of this onset larger than the beam resolution, the measurements are continued on a new freshly deposited film. Thus, the present experiments can be considered as conducted under essentially charge-free conditions. In the present ESD study, two series of thickness dependence measurements were performed. In a first series, films of ∼125 monolayers (ML) of I, II, and III (Figure 1) were condensed on the platinum substrate held at a temperature of ∼27 K. The second series of experiments involved adsorption of 0.1-1.5 ML of I, II, and III on a 10-ML Ar film. The Ar spacer layer served to minimize the effects of the image charge from the metal on desorbing anions. It also allowed the modification of or stabilization of MES contributions and estimation of their magnitude. The samples were prepared in a gas-handling manifold and condensed onto the substrate from the vapor phase. During deposition, the condensing vapors were analyzed for impurities by a residual gas analyzer. The film thickness was determined as previously described17 by a gas volume-expansion method with an uncertainty of (50% and a reproducibility of 5-10%. Samples of I (99.9+%), II (99%), and III (99%) were purchased from Aldrich and introduced in the manifold at the stated purity, where they were subjected to several in situ degassing cycles before deposition. The purity of the Ar gas (Matheson) was 99.9995%. The thickness of Ar films was further verified by measuring quantum interference structure in the transmitted current.18 Contamination from water was less than 0.005% in I, whereas in II and III it was less than 0.02%. At low coverages, perdeuterotetrahydrofuran (I-d8) was used in place of I to avoid possible H- contributions to the ESD signal from trace impurities containing hydrogen. The stated isotopic purity of I-d8 was 99.5% (Aldrich). However, because of the mass dependence of ion transmission in the mass spectrometer, a quantitative comparison between the relative intensity of Hand D- ESD yields was not possible.

Electron-Stimulated Desorption of H-

Figure 2. Comparisons of H- ESD yields produced by impact of 0to 40-eV electrons on 6-ML-thick films of I, II, and III. The smooth solid lines serve as guides to the eye. The curve baselines have been shifted vertically for clarity.

Figure 3. Comparison of H- and D- ESD yield functions produced by impact of 0- to 40-eV electrons on 1.5-ML films of I and I-d8 deposited on Ar spacer films. The smooth solid lines serve as guides to the eye. The curve baselines have been shifted vertically for clarity.

III. Results and Discussion The energy dependence of the H- ESD yield from 6-MLthick films of I, II, and III is shown in Figure 2. Similar Hyield functions are obtained for small coverages of I, II, and III on an Ar spacer layer, as shown in Figure 3 for 1.5 ML of I and I-d8 condensed on 12 and 10 ML of Ar, respectively. The comparison in Figure 3 further indicates that, except for the improved signal-to-noise ratio in the H- ESD yield, both

J. Phys. Chem. B, Vol. 104, No. 19, 2000 4713 curves display nearly identical behavior. Thus, any H- signal arising from possible contaminants does not seem to contribute significantly to the observed yields. For Ei between about 6 and 14 eV, at least one resonance in each of the three model compounds is discernible in the ESD yield function of Figure 2. This structure has maximum signal intensity near Ei ) 10 eV, and has been associated with the formation of a core-excited shape resonance leading to selective dissociation of endocyclic R-CH bonds.9 The DEA H- structure observed at 10 eV is similar to that measured from other condensed cyclic hydrocarbons,1,12 as well as linear saturated and unsaturated hydrocarbons.12 In some of these cases the addition of a group or substitution of heteroatoms in the benzene ring results in a substantial reduction in the H- desorption yield in comparison to benzene. Clearly, this is not the case for the derivatives of I. Owing to the strong similarity of the H- desorption profiles and the peak energy of 10 eV for I, II, and III, we conclude that the majority of the anion yield for all three systems studied arises from an electron resonance associated with electron attachment to the furan ring. For compounds II and III, a second peak is also observed in the H- ESD yield function and appears as a weak shoulder superimposed on the low-energy side of the 10 eV structure, displaying an onset near 6.7 eV and a maximum around 7.3 eV. In A, we noted the apparent lack of this 7.3-eV feature in the H- ESD yield function of III from overlap with the stronger 10-eV signal. In Figure 2 of the present study, this same feature is resolved in the H- ESD yield from III. Thus, on the basis of our previous studies, the 7.3-eV peak in III is attributed to DEA to the OH substituent whose corresponding parent state may be similar to that observed in CH3OH, where the 7.3-eV resonance originating from the hydroxyl group was previously assigned11,19 to a 2A" Feshbach resonance. In the incident electron energy region between about 16 and 28 eV, we observe a single broad structure in Figures 2 and 3 superimposed on a monotonically increasing baseline. The baseline signal increases approximately linearly with Ei from threshold, suggesting that it originates from nonresonant DD. This same general behavior is observed in the ESD curves for each of the three model compounds. The broad high-energy feature centered near Ei ) 23 eV, on the other hand, exhibits behavior characteristic of an electron resonance. For example, as a function of coverage, its intensity ratio with that of the low-energy 10-eV resonance is constant. This is best observed in the thickness dependence measurements of the D- ESD yield from monolayer and submonolayer coverage of I-d8 deposited on a 10-ML Ar film, and the H- ESD yield from films of III deposited directly on the platinum substrate, shown in Figure 4. In each case, the intensity of the 23-eV feature appears to grow at about the same rate as that of the 10-eV feature. In addition, for all coVerages studied, the intensity of the 23-eV structure remains always larger than the 10-eV intensity, even for submonolayer adsorption on a thick Ar substrate, where the image charge acting on the desorbing ion and MES should be significantly reduced. Furthermore, in the case of constant film thickness (Figure 4a), MES is constant in the Ar spacer, so that at submonolayer coverage of I, changes in the magnitude of the 23-eV peak cannot be attributed to MES. Anion formation also occurs predominately by single-electron scattering processes in III under the condition of very low coverage, shown in Figure 4(b), where the 23-eV signal lying above the DD baseline is seen to carry notable intensity. Thus, these observations suggest that, in addition to direct DD contributions, a significant portion of the H-(D-) signal in the 23-eV peak is due to the formation

4714 J. Phys. Chem. B, Vol. 104, No. 19, 2000

Antic et al.

Figure 5. Curve-fitted yield distribution for the D- ESD yield produced by impact of 0- to 40-eV electrons on a 1.7-ML film of I-d8 deposited on a 10-ML film of Ar. The smooth solid line is the Gaussian fit to the data. Individual Gaussian functions are represented by dashed lines.

Figure 4. Comparisons of anion ESD yield curves produced by impact of 0- to 40-eV electrons on condensed I-d8 and III. (a) D- ESD signal collected as a function of I-d8 coverage (expressed in ML) of a 10-ML Ar spacer film. (b) H- ESD signal collected from films of III of varying thicknesses deposited on the Pt substrate. The smooth solid lines serve as guides to the eye. The curve baselines have been shifted vertically for clarity.

of a transient anion followed by its decay into the DEA or DD channels. However, MES does not seem to contribute appreciably to the magnitude of the 23-eV peak. This latter behavior is in contrast to what is generally observed for broad peaks appearing above the DD threshold in ESD curves, where it has been shown3-5 that such features are largely the result of MES within the bulk solid before electron attachment. Above the electronic excitation threshold of the condensed solid, the incident electron may suffer, in addition to multiple energy losses due to vibrational and phonon excitation, energy losses due to electronic excitation. If the energy of the electron, after electronic excitation, is sufficient to form a NIS of a target molecule, it too may contribute to the anion yield. In this case, the DEA peak is shifted at higher energies. For example, a priori the peak at 23 eV could arise from a 23-eV electron which, after having lost 13 eV to electronic excitation, forms the NIS at 10 eV. Sambe et al.4 had clearly demonstrated this phenomenon for O- ESD yields measured from O2/Ar/Pt films by showing that in the absence of high-energy DEA resonances, features above the monotonically increasing DD baseline are largely due to electron energy losses in the rare-gas substrate (or O2 overlayer) before electron attachment to O2 resonances at lower electron energies. In that case, the intensity of the high-energy structure is always significantly less than that of the DEA resonance at lower energy. Evidence of MES contributions was even more convincing in the work of Azria et al.,5 who observed similar behavior in the Cl- ESD yield function from pure disorder physisorbed films of Cl2 as well as in the KE distributions of desorbing Cl- ions. Simpson and co-workers6,20 also argued that the broad feature they observed between 18 and 32 eV in the D- and D2 yields from condensed water must include the effect

of resonances, because DD alone is expected to increase monotonically with energy from threshold. In the present case, for DEA following multiple inelastic scattering to occur at Ei ) 23 eV, the electron must lose between 16 and 11 eV to electronic excitation before electron attachment in the lower-energy resonance region. Such high-energy losses are well above the ionization threshold (∼9.8 eV21-23) for I, II, and III, where information on doubly excited states is not available. In any case (see Figures 2 and 4b), if the 23-eV feature were to derive largely from MES within the molecular solid, its signal intensity should not be larger than that at 10 eV and should be significantly diminished for submonolayer coverage (i.e., under single scattering conditions). To the contrary, for submonolayer films of I, II, or III deposited directly on the Pt substrate or on a thick Ar film, this feature appears with approximately the same intensity relative to the 10-eV DEA signal. Hence, the largest contribution to the magnitude of the 23-eV peak does not arise from MES followed by DEA, given that it remains essentially unchanged under a variety of deposition conditions. Separating the direct resonant contributions (i.e., DEA without MES and resonance decay into the DD channel) from DD was accomplished by fitting the experimental curves. We found that three Gaussian functions centered at 10, 23, and about 40 eV fit the curves reasonably well. Figure 5 shows a fitted experimental D- ESD curve representative of all fitted curves. The DD signal, which comprises the baseline above an Ei of about 14 eV, is approximately linear above 25 eV. Figure 6 shows the I-d8 thickness dependence of the deconvoluted features, such as those in Figure 5, with a constant (10-ML) Ar spacer thickness. At 10, 23, and 40 eV, the D- signal exhibits, within experimental error, a linear behavior with thickness, corroborating our conclusion up to at least 1.6 ML that MES within the adsorbate is negligible. The influence of the metal on the DD signal is seen in Figure 7, where we plot the energy-integrated H- signal at 40 eV obtained from films of II of varying thickness up to about 14 ML. Initially, the H- DD signal increases rapidly, much faster than the 10-eV signal, reaches a maximum near 6 ML, and diminishes at higher coverages. In their study of the image charge influence on the O- yield from O2 films, Sambe et al.4 demonstrated that at small coverages the metal substrate would

Electron-Stimulated Desorption of H-

Figure 6. Thickness dependence of the D- yields produced by ESD from I-d8 condensed on a constant-thickness 10-ML Ar spacer film. The incident electron energies are indicated in the figure. The curve labeled 10 eV has been multiplied by 5 to bring it on scale with the remaining curves. The solid lines serve as guides to the eye.

J. Phys. Chem. B, Vol. 104, No. 19, 2000 4715 beyond 5 ML. This behavior gives a good indication that this resonance decays into energetically similar parent states, leading to the same dissociation limit as that observed in the nonresonant DD portion of the H-(D-) ESD signal. On the other hand, at small coverage of the Ar spacer in Figure 6, the 23-eV signal tracks well the DEA signal found at 10 eV. Thus, because the major contribution to the 23-eV peak does not involve MES, we conclude that its magnitude arises essentially from resonance decay into DD, with possibly small contributions from direct DEA. The high energy of the 23-eV feature makes the assignment of the electronic state or states involved in the resonance difficult. Experimental data on the electronic states for I are limited to electronic excitations in the Rydberg series below the first ionization potential near 9.8 eV27,28 and appear to be nonexistent for II and III. Nevertheless, considering the relatively large width of the 23-eV feature (i.e., ∼6 eV) and the general disappearance of Rydberg states above the ionization limit in the condensed phase, we suggest that overlapping coreexcited valence-type resonances are probably involved in H-(D-) desorption around this energy. Finally, we may also suggest that the resonant state(s) responsible for the 23-eV feature has its origin in the furan ring of I, II, and III, as there is very little change in the basic shape, energy, width, and peak position of this high-energy structure regardless of the parent molecule. IV. Summary and Conclusions

Figure 7. Thickness dependence of the H- yields produced by ESD from II deposited on the Pt substrate. The solid lines serve as guides to the eye. The curves were recorded at incident electron energies of 10, 23, and 40 eV.

enhance the DD contributions with respect to the DEA signal. As explained in the Introduction, this is due to additional polarization energy that arises from the creation of two opposite charges from a neutral state in the case of DD. However, as the film thickness increases, the contribution of this effect to the DD signal also increases, but only up to a point, beyond which the influence of the underlying metal substrate becomes negligible. The same behavior is generally observed in the HDD yield for most hydrocarbon films,12 as well as for films of I and III, in the present experiment. The thickness dependence of H- DEA yield at 10 eV, on the other hand, is characterized by a gradual increase in signal for coverages between zero and 4 ML, and approaches an asymptote near 6 ML. This behavior is a general characteristic of DEA yields from thin films, and has been observed in H- yield functions from condensed hydrocarbon films,12 in O- yields from submonolayer O2 deposited on rare-gas films of various thickness,4 and D- yields from amorphous and crystalline D2O.26 As seen in Figure 7, the 23-eV H- ESD signal exhibits a behavior that is intermediate between those of the DD and DEA processes. Similarly to the DD signal, it rises rapidly at small coverages and diminishes

In the present study, we have examined the low-energy electron-induced degradation of a series of model compounds that are representative of the deoxyribose backbone in DNA, that is, tetrahydrofuran (I), 3-hydroxytetrahydrofuran (II), and R-tetrahydrofurfuryl alcohol (III). For incident electron energies between 0 and 40 eV, we observe two resonance structures in the H- ESD yields from physisorbed films of I, and three from films of II and III. The first structure common to each molecule occurs near Ei ) 10 eV, and has previously been attributed to a core-excited shape resonance leading to H- production via DEA. In II and III, a lower-energy resonance near 7.3 eV associated with H- production via DEA to the hydroxyl group has also been identified. Another resonance in the H- ESD yields common to all three species is found around 23 eV (i.e., aboVe the DD threshold). By measuring the thickness dependence of the H-(D-) ESD signal, we found that this high-energy structure shows a behavior intermediate between that of DEA and resonance decay into DD. Its behavior in the low-coverage region was nearly identical to that of the resonance near 10 eV. On the other hand, at higher coverages the 23-eV H- signal resembled anion production via DD. Combining both characteristics, we concluded that the transient anion (or anions) formed near 23 eV decays principally by electron emission into an electronically excited state (or states) that dissociates into H- and the corresponding radical cation and possibly to a smaller extent into the DEA channel. Acknowledgment. One of us (D.A.) acknowledges the support of the Gouvernement of Que´bec, Ministe`re de l’EÄ ducation, in the form of a postdoctoral fellowship (Program Que´be´cois de Bourse d’Excellence). The authors are indebted to M. Michaud for helpful comments. This research is supported by the Medical Research Council of Canada via grant no. 14502. References and Notes (1) Sanche, L. Scanning Microsc. 1995, 9, 619-656. (2) Sanche, L. Phys. ReV. Lett. 1984, 53, 1638-1641.

4716 J. Phys. Chem. B, Vol. 104, No. 19, 2000 (3) Sambe, H.; Ramaker, D. E.; Parenteau, L.; Sanche, L. Phys. ReV. Lett. 1987, 59, 505-508. (4) Sambe, H.; Ramaker, D. E.; Parenteau, L.; Sanche, L. Phys. ReV. Lett. 1987, 59, 236-239. (5) Azria, R.; Parenteau, L.; Sanche, L. J. Chem. Phys. 1987, 87, 22922296. (6) Simpson, W. C.; Sieger, M. T.; Orlando, T. M.; Parenteau, L.; Nagesha, K.; Sanche, L. J. Chem. Phys. 1997, 107, 8668-8677. (7) Klyachko, D. V.; Huels, M. A.; Sanche, L. Radiat. Res. 1999, 151, 177-187. (8) Dugal, P. C.; Huels, M. A.; Sanche, L. Radiat. Res. 1999, 151, 325-333. (9) Antic, D.; Parenteau, L.; Lepage, M.; Sanche, L. J. Phys. Chem. B 1999, 103, 6611-6619. (10) Michaud, M.; Fraser, M.-J.; Sanche, L. J. Chem. Phys. 1994, 91, 1223-1227. (11) Parenteau, L.; Jay-Gerin, J. P.; Sanche, L. J. Phys. Chem. 1994, 98, 10277-10281. (12) Rowntree, P.; Parenteau, L.; Sanche, L. J. Phys. Chem. 1991, 95, 4902-4909. (13) Rowntree, P.; Sambe, H.; Parenteau, L.; Sanche, L. Phys. ReV. B 1993, 47, 4537-4554. (14) Huels, M. A.; Parenteau, L.; Michaud, M.; Sanche, L. Phys. ReV. A 1995, 51, 337-349.

Antic et al. (15) Rowntree, P.; Parenteau, L.; Sanche, L. J. Chem. Phys. 1991, 94, 8570-8576. (16) Sanche, L.; Descheˆnes, M. Phys. ReV. Lett. 1988, 61, 2096-2098. (17) Sanche, L. J. Chem. Phys. 1979, 71, 4860-4882. (18) Perluzzo, G.; Sanche, L.; Gaubert, C.; Baudoing, R. Phys. ReV. B 1984, 30, 4292-4296. (19) Curtis, M. G.; Walker, I. C. J. Chem. Soc., Faraday Trans. 1992, 88, 2805-2810. (20) Kimmel, G. A.; Orlando, T. M. Phys. ReV. Lett. 1996, 77, 3983. (21) Tasaki, K.; Yang, X.; Urano, S.; Fetzer, S.; LeBreton, P. R. J. Am. Chem. Soc. 1990, 112, 538-548. (22) Bremner, L. J.; Curtis, M. G.; Walker, I. C. J. Chem. Soc., Faraday Trans. 1991, 87, 1049-1055. (23) Kim, H. S.; Yu, M.; Jiang, Q.; LeBreton, P. R. J. Am. Chem. Soc. 1993, 115, 6169-6183. (24) Sanche, L.; Michaud, M. Phys. ReV. B 1984, 30, 6078-6092. (25) Huels, M. A.; Parenteau, L.; Sanche, L. J. Chem. Phys. 1994, 100, 3940-3956. (26) Simpson, W. C.; Orlando, T. M.; Parenteau, L.; Nagesha, K.; Sanche, L. J. Chem. Phys. 1998, 108, 5027-5034. (27) Davidson, R.; Høg, J.; Warsop, P. A.; Whiteside, A. B. J. Chem. Soc., Faraday Trans. 2 1972, 68, 1652-1658. (28) Doucet, J.; Sauvageau, P.; Sandorfy, C. Chem. Phys. Lett. 1972, 17, 316-319.