Products and Reaction Sequences in Tetrahydrofuran Exposed to Low

12512

J. Phys. Chem. B 2006, 110, 12512-12522

Products and Reaction Sequences in Tetrahydrofuran Exposed to Low-Energy Electrons Claudia Ja1 ggle and Petra Swiderek* UniVersita¨t Bremen, Fachbereich 2 (Chemie/Biologie), Leobener Strasse/NW 2, Postfach 330440, 28334 Bremen, Germany

Simon-Philippe Breton, Marc Michaud, and Le´ on Sanche Groupe en Sciences des Radiations, De´ partement de Me´ decine Nucle´ aire et Radiobiologie, Faculte´ de Me´ decine, UniVersite´ de Sherbrooke, Sherbrooke Que´ bec, Canada J1H 5N4 ReceiVed: March 8, 2006

Electron-stimulated reactions in solid films of tetrahydrofuran (THF), condensed on Kr spacers deposited on a Pt substrate, or directly onto the substrate, were induced and monitored simultaneously with use of highresolution electron-energy-loss spectroscopy in the ranges of vibrational and electronic excitations. The spectra of the molecular films obtained after a certain time of exposure to electrons at incident energies of 14 and 15.5 eV were analyzed and different products were identified. Besides an aldehyde, which is the main product, olefins, conjugated olefins, as well as CO were identified. Closer investigation of the reactions of propionaldehyde, as a model aldehyde, demonstrates that CO appears in THF as a secondary product (i.e., from the intermediate aldehyde). On the basis of the cross sections for the formation of an aldehyde from THF, of CO from propionaldehyde, and for the loss of propionaldehyde under electron impact, the reaction sequences were evaluated with the help of a kinetic model. This analysis suggests that some CO could also be formed directly from THF (i.e., without involvement of an intermediate aldehyde).

1. Introduction Chemical reactions induced by low-energy electrons (LEE) play a crucial role in processes for surface modification,1-5 radiation damage in biological matter,6,7 and possibly even atmospheric processes.8 As such processes typically take place under exposure to high-energy radiation such as X-rays or kiloelectronvolt electrons, secondary electrons with energies below 20 eV are produced abundantly.9 These LEE are further responsible for an important part of the chemistry that takes place in the irradiated material. Therefore, a comprehensive picture of the products being created and the reaction sequences occurring in molecular materials under exposure to LEE is required. In a solid material, the effect of LEE bombardment is spatially restricted to a region near the vacuum-solid interface owing to the limited mean free path of electrons within the solid.10 Furthermore, when a thin film deposited on a metal surface is exposed to electrons, reactive processes may be quenched near the metal, which again limits the region where modifications occur to the topmost layers.11 Consequently, low amounts of product molecules have to be detected in the analysis of the molecular film after a certain time of exposure to electrons (i.e., the reaction mixture). This requires techniques that are both sufficiently sensitive and able to distinguish the different products that may be formed. Electron-energy-loss (EEL) spectroscopy is a highly sensitive and surface selective technique. Its analytic capabilities in the ranges of both vibrational and electronic excitations can be used to monitor reactions induced by LEE.12-15 Using this approach absolute cross sections for the formation of CO within solid films of acetone16 and methanol,13 of propene within films of cyclopropane,17 and * To whom correspondence should be addressed.

of aldehyde within solid films of tetrahydrofuran (THF) have been obtained.15 While these previous studies have shown that not only a qualitative but also a quantitative investigation of electroninduced reactions by EEL spectroscopy is feasible, the analysis so far has been restricted to one specific product. On the other hand, it is often obvious that exposure to LEE yields a mixture of reaction products. For example, in addition to isomerization, oligomer formation takes place in cyclopropane18 and the production of CO from acetone16 implies that hydrocarbon products are formed as well. As an extension to the previous study on THF,15 this work concentrates on a more detailed identification of products formed under exposure to LEE. This includes secondary reactions, i.e., the degradation of aldehydes that have been identified as immediate products of THF. More generally, this analysis gives us the opportunity to explore how much information on the products of LEE-induced reactions in a molecular solid can in principle be obtained by EEL spectroscopy. To this end, the effect of exposing a multilayer solid film of THF to electrons with incident energies (E0) of 14 and 15.5 eV is investigated. Products are identified by comparing the EEL spectra of the reaction mixtures to those of a set of representative compounds, and also to model spectra obtained by computing a weighted sum of the spectra of the pure compound and those of the suspected products. Furthermore, the spectra of the reaction mixtures are decomposed by difference techniques into contributions from the identified products and the residual spectral intensities are discussed in terms of additional products. Finally, reaction cross sections for the consecutive electron-induced reactions THF f aldehyde and further aldehyde f CO as well as for the overall loss of aldehyde are determined and discussed.

10.1021/jp0614291 CCC: $33.50 © 2006 American Chemical Society Published on Web 06/07/2006

Electron-Stimulated Reactions in Solid Films of THF 2. Experimental Section The experiments were performed with two different highresolution electron-energy-loss (HREEL) spectrometers. Vibrational spectra were recorded with the Bremen apparatus that contains an HREEL spectrometer with cylindrical deflectors19 incorporated in a µ-metal-shielded UHV chamber reaching a base pressure of 10-10 Torr by the combined action of an ion pump and a titanium sublimation pump. This setup has been described in detail elsewhere.20 The monochromator, which can be rotated between 14° and 120° with respect to the normal to the sample, was set to 60° for the present experiments. The analyzer was fixed at 60° at the opposite azimuth. The resolution was adjusted between 10 and 15 meV full width at halfmaximum (fwhm) for transmitted currents on the polycrystalline Pt substrate ranging from 0.2 to 0.3 nA. The incident energy E0 was calibrated to within (0.2 eV, using the onset of the transmitted current, and further corrected for cutoff effects of the lenses by putting a retarding field on the target.20 Gases or vapors are introduced via a gas-handling manifold consisting of precision leak valves and a small calibrated volume where the absolute pressure is measured with a capacitance manometer. For each film deposition a calibrated amount of gas or vapor is leaked via a stainless steel capillary whose end is located just in front of the substrate held at 35 K by a closedcycle Helium refrigerator (Leybold Vacuum). Prior to each deposition the substrate is cleaned by resistive heating to an orange glow. Tetrahydrofuran (THF), acetaldehyde (AA), and hexane (HX) were purchased from Fluka with a stated purity of >99.5%, 99.5%, and spectroscopic grade, respectively, and ethylene (ET, 99.95%) and propene (PR, 99.98%) were obtained from Messer Griesheim. Propionaldehyde (PA, 99+%) was obtained from Acros Organics and butyraldehyde (BA, >99%) from Merck. All compounds were used without further purification. Liquid samples were subject to degassing by repeated freeze-thaw cycles under vacuum. The thickness of the deposited films was estimated from the calibrated amount of vapor needed to deposit a monolayer of cyclopropane14 and benzene,20 taking into account the differences in molecular size. Furthermore, the thickness for the present experiments was chosen in a regime where signals in the vibrational EEL spectra saturate, thus providing evidence that the multilayer film is sufficiently thick to neglect the effects of the underlying substrate in the analysis. Electronic spectra were recorded in Sherbrooke with a hemispherical EEL spectrometer21 located inside a µ-metal UHV chamber pumped down to a pressure of 6 × 10-11 Torr with the combined action of an ion pump and a liquid-N2 cooled titanium sublimation pump. Double-zoom electrostatic lenses located at the exit of the monochromator and at the entrance of the analyzer ensure a fixed focusing of the beam on the target as well as a constant beam size over a wide range of primary energies (e.g., 1-25 eV). The monochromator, which can be rotated from 8° to 80° with respect to the normal to the sample, was set at 8°, and the analyzer fixed at 45° at the opposite azimuth. The combined resolution of the selectors was set to 28 meV fwhm for a current of I0 ) (1.2 ( 0.2) nA incident on the substrate. The incident electron energy E0 was calibrated within (0.1 eV with respect to the vacuum level by measuring the threshold of the electron current transmitted through the samples. Gases or vapors are introduced via a gas handling manifold and leaked via a stainless steal capillary whose end is located just in front of a Pt(111) single crystal. This target is mounted on a sample manipulator,22 which allows azimuthal rotation,

J. Phys. Chem. B, Vol. 110, No. 25, 2006 12513 flip, and translations of the sample along the X, Y, and Z directions, and is held at a temperature of 25 K via a flexible copper braid attached to the cold end of a closed-cycle helium refrigerator (APD Cryogenics Inc., Allentown, PA). Crystal cleaning is achieved by resistive heating at 1100 K and sequences of Ar+ sputtering followed by annealing and heating in the presence of oxygen at about 900 K. THF, AA, PA, and BA were purchased from Aldrich Chemical Ltd. with stated purities of 99.9%, 99.5+%, 97%, and 99.5+%, respectively, and used without further purification after degassing by repeated freeze-thaw cycles under vacuum. The number of condensed layers in a deposited film was estimated, as described previously,23 to (10% from the calibrated amount of vapor needed to deposit a monolayer, assuming the same sticking coefficient and growth for the adlayers. To prevent possible decomposition of the molecules on the Pt crystal, as well as to rule out any insidious effect due to the proximity of the metal surface, a six-layer film of Kr (99.995%, Praxair Inc.) was systematically used as a spacer between a deposited film and the Pt substrate. For experiments aiming at determining cross sections, nine spectra were recorded at different positions of each deposited film defining a 3 × 3 rectangular array on the target (positions distant by 1.5 mm along the X direction and 1 mm along the Y direction). The positions on the target were always visited in the same order, beginning at the center, then jumping to the four corners, and finishing with the four sides. With this partitioning no noticeable difference was found between the EEL spectra measured for the first five positions, furthest from one another, and for the last four. Such a strategy allowed, with a limited number of film preparations, us to record the spectra over a wider EEL range with a better signal-to-noise ratio. Although the incident current was constant during the course of an experiment, slight changes could be monitored from one experiment to another. So, considering the scattered intensity to be directly proportional to the incident current, the intensity scale of each spectrum was systematically normalized to a reference current of 1.2 nA. Typically the current impinging on the sample in the Bremen instrument was five times lower and covered a considerably larger area than that in the Sherbrooke apparatus. An exact value for the beam size in the Bremen instrument cannot be given as its dimensions could only be estimated in one direction by displacing the sample with respect to the spectrometer and observing the drop in current as the beam reaches the edge of the sample. The measurement of the beam cross section in the Sherbrooke setup is described in detail in section 3.3. Because of the lower current and the larger area, electron exposure times in the case of the vibrational spectra (i.e., recorded on the Bremen instrument) were much longer compared to the EEL experiments concerning the range of the electronic excitations performed with the Sherbrooke setup. Nonetheless, the longer recording times allow us to resolve clearly the progress of changes in the spectra under exposure as each spectrum can be recorded with a satisfactory signal-to-noise ratio. 3. Results and Discussion 3.1. Vibrational Spectra of THF Exposed to 15.5-eV Electrons. The vibrational EEL spectrum of a freshly deposited 10-layer film of THF condensed on the Pt substrate recorded at E0 ) 15.5 eV is shown as the top trace in Figure 1a. A comprehensive assignment of the vibrational bands of THF has been given previously.24 The most intense bands are identified by comparison with those results as an overlap of the ring bending and CH2 rocking modes (ν16 and ν32) at 76 meV, the C-C stretching modes (ν13 and ν30) at 114 meV, an overlap of

12514 J. Phys. Chem. B, Vol. 110, No. 25, 2006

Figure 1. (a) Vibrational electron-energy-loss (EEL) spectrum at an incident electron energy E0 ) 15.5 eV of a fresh 10-layer film of THF condensed on a polycrystalline Pt substrate (THF), spectrum of the same film after 17.5 h of irradiation with electrons at the same energy (THFirr), and difference between the THFirr and THF spectra (see text). Each spectrum was recorded during 53 min. (b) Vibrational EEL spectra recorded at E0 ) 15.5 eV of fresh multilayer films of acetaldehyde (AA), propionaldehyde (PA), butyraldehyde (BA), hexane (HX), and ethylene (ET) condensed on polycrystalline Pt and vibrational EEL spectrum at E0 ) 15.0 eV of propene (PR) condensed on the same substrate. Dashed lines mark the positions of the CdO und CdC stretching vibrations.

different C-H deformation modes (ν10, ν26, ν8, and possibly others) centered around 157 meV, the CH2 bending modes (ν6 and others) at 181 meV, and C-H stretching modes at 364 meV. The spectrum of the same film irradiated for ∼1100 min with 15.5-eV electrons, shown as the middle trace in Figure 1a, exhibits characteristic changes relative to that of fresh THF. These changes result from a decreasing intensity of the vibrational EEL features of THF and partly overlapping increasing bands of new compounds produced by exposure to the electron beam. A difference spectrum, shown as the bottom curve in Figure 1a, reveals the changes more clearly. This curve was obtained by subtracting from the spectrum of electron exposed THF a fraction of the spectrum of fresh THF that was adjusted until negative signals of THF just vanished. Depletion of THF under exposure to 15.5-eV electrons is most obvious from the decrease of the intense bands at 114 and 157 meV in the vibrational spectrum. A similar decline in intensity is not observed for the other THF bands because of the superposition with the product bands as identified from the difference spectrum. More specifically, new bands in the 5060 meV range and around 90 meV lead to an apparent smearing out of the THF ring bending signal at 76 meV. In addition, new bands appear around 130 and 176 meV and within the 200-220 meV range, the latter of which covers the vibrational stretching frequencies of CdO and CdC bonds.25 This suggests that CdO or CdC unsaturated products are formed in a film of THF under exposure to 15.5-eV electrons. To attempt an identification of these products, the spectrum of exposed THF

Ja¨ggle et al. along with the difference spectrum (Figure 1a) was compared to the spectra of multilayer films of different aldehydes and olefins as well as a saturated hydrocarbon (HX) of similar thickness as THF and recorded at about the same E0, as shown in Figure 1b. Ketone species were excluded from this comparison on the basis that the C-O bond is expected to be the most reactive in THF7,15 and therefore should break more easily following the excitation of an electronic state or the formation of a transient anion state, thus producing a chain terminating with oxygen. The spectra of all aldehydes show a characteristic CdO stretching band with a maximum at 212 meV that agrees with the new band appearing between 200 and 220 meV in the spectrum of exposed THF. In addition, the intense C-H deformation band25,26 at 176 meV explains the intensity increase observed at the same energy loss in the spectrum of exposed THF. Within the energy-loss range below 100 meV, the picture is less clear, except that the lack of pronounced peaks in the spectrum of exposed THF may argue against the presence of AA and PA in favor of BA. This is also plausible since the formation of BA requires less extensive bond reorganization. Although the CdO stretching band of the aldehydes falls well within the range of the new band between 200 and 220 meV, the latter is still somewhat broader as better seen from the difference spectrum in Figure 1a. This suggests that at least one additional product contributes to the intensity. An overlap with a CdC stretching band, as exemplified by the spectra of similarly grown films of ET and PR in Figure 1b, can explain this broadening. Furthermore, a positive signal in the C-H stretching region of the difference spectrum also compares well with the C-H stretching band of ET and the high-energy tail of the C-H stretching band of PR. The latter is broader than the C-H stretching band of ET and the aldehydes because of overlapping contributions from saturated and unsaturated C-H bonds. The mere comparison of the difference spectrum with that of the different aldehydes and olefins suggests the presence of both species in exposed THF. To support this interpretation, the spectrum of each aldehyde was added to that of fresh THF with a weighting factor that was adjusted until the spectrum of exposed THF was reproduced closely by the sum. These sums, for which the best possible agreement was achieved, are presented along with the spectrum of exposed THF in Figure 2a. Small remaining dissimilarities persisting within specific spectral regions, which are better revealed in the curves of Figure 2b obtained by subtracting the weighted sums (Figure 2a) from the spectrum of exposed THF, are ascribed to products other than the aldehyde. Most importantly, a band between 120 and 140 meV as well as a residual intensity within the 50-60 meV range and around 90 meV are seen in the difference spectra and are therefore not explained by any of the aldehydes. In addition, a small residual signal at 203 meV as well as in the range of C-H stretching vibrations of unsaturated hydrocarbons (around 380 meV) again hint toward formation of CdC bonds. The band within the 120-140 meV range can only partly be explained by the strong olefin CH2 wagging band25 between 115 and 120 meV. Consequently, the residual intensity around 130 meV as well as the origin of the two new bands at lower energy remain to be assigned. So far, it is not clear whether the residual intensity in the difference plots of Figure 2b, which can in part be traced back to the formation of olefin species, stems directly from THF under electron exposure, or indirectly from a reaction of the aldehydes accumulated under prolonged exposure. To shed light

Electron-Stimulated Reactions in Solid Films of THF

Figure 2. (a) Vibrational EEL spectrum of a multilayer film of THF condensed on a polycrystalline Pt substrate recorded at E0 ) 15.5 eV after 17.5 h of exposure to electrons at the same energy (THFirr, same as Figure 1). Weighted superpositions (lower curves) of the spectrum of fresh THF (Figure 1) and those of different aldehydes (THF + AA, THF + PA, THF + BA) yielding the best overall fit to THFirr. (b) Difference between the THFirr spectrum and the THF + AA, THF + PA, and THF + BA superpositions (see text).

Figure 3. (a) Vibrational EEL spectra at E0 ) 15.5 eV of a fresh multilayer film of PA condensed on a polycrystalline Pt substrate (PA), and of the same film after 19.3 h of irradiation with electrons at the same energy (PAirr). Each spectrum was recorded during 54 min. (b) Difference between the PAirr and PA spectra (see text).

on this problem, the effect of electron exposure on pure aldehyde films was also investigated. Since the results were similar for the different aldehydes, PA only is shown here as a representative example. Figure 3a shows the vibrational EEL spectra of both a freshly deposited multilayer film of PA (upper trace) and of the same film following ∼1200 min of irradiation with 15.5-eV electrons (lower trace). The latter exhibits characteristic changes indicating that PA reacts also upon electron exposure. The changes are again better visualized by a difference spectrum in Figure 3b obtained in the same way as for THF in Figure 1. Similar to THF, the spectrum of PA loses its structure in the energy-loss range below 100 meV because of the emergence of new bands around 63 and 98 meV. Besides, a general increase in intensity with no clear structure is observed in the range from 128 to 165 meV. More characteristic is the shift of the original

J. Phys. Chem. B, Vol. 110, No. 25, 2006 12515

Figure 4. (a) Vibrational EEL spectrum at E0 ) 15.5 eV of a multilayer film of THF condensed on a polycrystalline Pt substrate after 17.5 h of irradiation with electrons at the same energy (THFirr, same as Figure 1). Weighted superpositions (lower curves) of the spectrum of fresh THF and those of aldehydes irradiated with 15.5-eV electrons during 20.1 h for AA (THF + AAirr), 19.3 h for PA (THF + PAirr), 14.8 h for BA (THF + BAirr), and those of fresh PA and ET (THF + PA + ET) yielding the best overall fit to THFirr. (b) Difference between the THFirr spectrum and the superpositions: THF + AAirr, THF + PAirr, THF + BAirr and THF + PA + ET (see text).

CdO stretching band to lower energy, as witnessed by the residual signal at 203 meV. Finally, the shift to higher energy and the broadening of the C-H stretching band hint again to the formation of a CdC bond akin to that in olefins. The latter two findings suggest that at least part of the olefin content in exposed THF may result indirectly from the ongoing electron exposure of the accumulating aldehyde product. Moreover, as shown in Figure 4a, the agreement between the spectrum of exposed THF (top) and the weighted sums (below) is improved somewhat when spectra of exposed aldehydes are used instead of those of fresh aldehydes [cf., Figure 2a]. This is again revealed more clearly by the difference spectra in Figure 4b, which were obtained by subtracting the weighted sums from the spectrum of exposed THF. In fact, a further intensity decrease results for both the CdC stretching and associated C-H stretching bands when using the spectrum of exposed BA. The 130-meV band, which, on the contrary, retains to a large extent its intensity, most probably reflects the presence of another direct product of exposed THF. To furnish further support for the formation of an olefin species, a weighted sum of the spectra of fresh THF, PA as a representative aldehyde, and ET as a representative olefin, performed with the aim of reproducing more closely the CdC and CdO stretching vibrations, is also included as the bottom curve in Figure 4a. While a good match with the spectrum of exposed THF is obtained in this range, the agreement at lower energies, where delocalized vibrations of the whole molecule play a more pronounced role, is not satisfactory. This underlines that more complex olefins than ethylene must be formed. While evidence for the formation of aldehydes and olefins in THF exposed to 15.5-eV electrons is found in the CdC and


Ja¨ggle et al.

Figure 5. Integral intensity within the energy-loss ranges from 120 to 140 meV and 200 to 225 meV of the vibrational EEL spectra of THF recorded at E0 ) 15.5 eV as a function of the exposure time. Each data set is presented normalized to its energy interval and shifted vertically with the first value being equal to zero. The intensity scale is the same for both data sets.

CdO stretching ranges, the residual intensity observed below 140 meV in all difference spectra is not as easily explained. It must be noted, however, that the new bands at 130 and 90 meV and between 50 and 60 meV coincide closely with lowfrequency vibrations of longer alkanes.25 As an example, a vibrational EEL spectrum of hexane (HX) is included in Figure 1b. Apart from the strong C-H stretching and deformation bands at 361 and 180 meV, the medium C-C stretching and CH2 rocking vibrational bands can be observed around 130 and 92 meV along with a steep rise in intensity below 73 meV. The latter two also cover the range in the spectrum of THF in which the intensity increases with electron exposure. This suggests that longer saturated alkane chains may exist in exposed THF. On the other hand, an attempt to reproduce more closely the spectrum of exposed THF by adding some HX to the weighted sum of THF, PA, and ET [bottom curve in Figure 4a] was not successful. This result, which is not shown explicitely here, indicates again that more complex products than the model compounds included in Figure 1 are also formed in exposed THF. To determine if another product responsible for the 130-meV band forms directly from THF under electron exposure, the integral intensity between 120 and 140 meV is plotted in Figure 5 as a function of exposure time along with that between 220 and 225 meV for the combined CdO and CdC stretching bands. For an easier visualization, each data set is presented normalized to its energy interval and offset vertically with the first value being equal to zero. Apart from an initial drop, the intensity within the 120-140 meV range increases to reach a maximum and then decreases slightly. At the same time, the combined CdO and CdC signal shows a much smaller initial drop and then increases continuously with no apparent decline. Assuming the formation of aldehydes only, which show a relatively low intensity in the two ranges under consideration, and considering that THF has a high intensity within the 120-140 meV range, which must drop when THF is consumed, the total signal increase in the latter range should be less pronounced than that between 200 and 225 meV. The observation that both signals increase at a similar rate does not support this assumption and therefore suggests the presence of an additional immediate product formed from THF. Direct formation of an olefin with its strong band around 120 meV may account for only part of the intensity in this spectral range. Also, the olefin contribution that was identified earlier as resulting indirectly from the accumulating aldehyde should not be significant during the early stages of exposure.

Figure 6. (a) Electronic EEL spectra of a four-layer film of THF deposited on a six-layer Kr spacer: (lower curve) recorded during 2 min at E0 ) 14 eV and same recording time but after (middle curve) 8 min and (upper curve) 18 min of exposure to 14-eV electrons. The vertical dashed lines show the integration boundaries for the determination of the cross section for aldehyde formation. (b) Electronic EEL spectra of four-layer films of different aldehydes (AA, PA, BA) deposited on a six-layer Kr spacer and recorded during 2 min at E0 ) 14 eV. (c) Electronic EEL spectrum of a five-layer film of propene (PR) deposited on a five-layer film of Ar and recorded at E0 ) 15 eV with an angle of incidence (Θ0) of 15° and an angle of analysis (Θs) of 45°.18 Electronic EEL spectrum of a 15-layer film of ethylene (ET) deposited on polycrystalline Pt and recorded at E0 ) 13.5 eV with Θ0 ) 15° and Θs ) 45°.27

The position of the band at 130 meV coincides also with C-O stretching bands known from a series of alcoholic species as well as those of radicals such as methoxy or ethoxy.25 A possible explanation is that after the initial opening of the THF ring, hydrogen migration at the resulting biradical would produce a species containing an OH group. Similar to previous results on methanol,13 this intermediate product in turn might undergo reactions by forming CO under electron exposure. This interpretation in terms of an intermediate product would also explain the observed decline in intensity after a certain exposure time. 3.2. Electronic Spectra of THF and PA Exposed to 14-eV Electrons. Figure 6a shows electronic EEL spectra in the energy-loss range from 3 to 7.5 eV recorded with 14-eV electrons incident on a four-layer film of THF condensed on a six-layer Kr spacer for different electron exposures at the same energy. The electronic spectrum of THF is characterized by an excitation band onset at about 6 eV, followed by a monotonic rise due to the presence of several overlapping broad electronic bands.7,15 The spectrum recorded during the first 2 min of exposure to 14-eV electrons (Figure 6a) already shows between 3.6 and 6 eV small energy-loss features due to the formation of products. For larger exposures, their intensities increase as reported previously.7,15 The electronic spectra of representative aldehydes and olefins, recorded under the same or similar conditions (see figure caption) are included for comparison in Figure 6, parts b and c. The spectra of the aldehydes show a common maximum at about 4.25 eV assigned to the overlapping 3nπ* and 1nπ* transitions (i.e., 3,1nπ* band), followed by a


Figure 7. (a) Electronic EEL spectrum of a four-layer film of PA deposited onto a six-layer Kr spacer, recorded during 2 min at E0 ) 14 eV. (b) Spectrum of the same film as in part a recorded after 18 min of irradiation with 14-eV electrons (PAirr). (c) Electronic EEL spectrum in the range of the lowest lying triplet state (a3Π) of CO.13,16 The vertical dashed lines show the integration boundaries for determination of the cross sections for the CO production.

minimum at about 5.2 eV and then a broad band around 6.3 eV ascribed to the 3ππ* excitation.15 The 3ππ* band in olefins has a maximum around 4.3 eV followed only by the onset of the intense 1ππ* band at 6.4 eV in PR18 and at 6.7 eV in ET.27 As can be seen, the excitation threshold in Figure 6a coincides with the 3.6-eV onset in both the 3,1nπ* band in simple aldehydes and the 3ππ* band in olefins with an isolated double bond. It must be noted here that formaldehyde, which has been considered as a potential product previously,7 is excluded from the analysis because the noticeable vibronic structure of the 3,1nπ* band that has been observed in previous spectra28 is not obvious in the spectrum of THF after electron exposure.15 At first sight, the production of both classes of compounds in THF could be responsible for the appearance of the new band at around 4.3 eV upon electron exposure. In compliance with our vibrational analysis, attempts were carried out to reproduce the spectra of exposed THF by a weighted sum of spectra of THF and the potential products. As all aldehydes have a similar electronic spectrum, this procedure is only shown for PA as a representative example. Also, since the vibrational spectra already revealed that PA also undergoes reactions upon electron exposure, we show in Figure 7a the spectrum of PA [same as Figure 6b] along with that of the same film after 20 min of exposure to electrons at E0 ) 14 eV in Figure 7b. Under electron exposure, PA clearly develops a CO signature consisting of a series of vibronic bands, ascribed to the excitation of the lowest lying triplet state (a3Π) of CO, with the major part of the intensity located between 5.9 and 6.7 eV, as shown in Figure 7c.13,16 At the same time, the intensity of the 3,1nπ* transitions decreases indicating that PA is consumed. The spectrum of THF exposed to 14-eV electrons during 12 min is shown as the top trace in Figure 8a. The best overall fit to this spectrum that could be obtained by adding a certain proportion of the spectrum of PA [Figure 7a] to that of THF [Figure 6a], both recorded at low electron exposure, is shown as the middle curve in Figure 8a. A difference spectrum, shown as the top curve in Figure 8b and obtained by subtracting the weighted sum containing PA (middle) from the spectrum of


Figure 8. (a) Electronic EEL spectrum of a four-layer film of THF deposited on a six-layer Kr spacer, recorded during 2 min at E0 ) 14 eV after 10 min of irradiation with 14-eV electrons (THFirr). Weighted superposition (THF + PA) of the spectrum of THF obtained during the first 2 min recording at E0 ) 14 eV [same as lower curve of Figure 6a] with the spectrum of PA [same as Figure 7a] during the first 2 min recording at E0 ) 14 eV, and corresponding superposition (THF + PAirr) with the spectrum of PAirr (not shown) recorded during 2 min at E0 ) 14 eV after 16 min of exposure to 14-eV electrons, all of which yield the best overall fit to THFirr. (b) Difference between the THFirr spectrum and the THF + PA and THF + PAirr superpositions. The vertical dashed lines show the different spectral regions as discussed in the text.

exposed THF (top), reveals that not all of the intensity increase in the spectrum of exposed THF results from the aldehyde, but that above 4.7 eV other products must contribute as well. This is already expected from Figure 8a where the minimum at 5.2 eV in the weighted sum including PA (middle) is absent in the spectrum of exposed THF (top). The same is found for the weighted sum containing the spectrum of PA exposed to 14eV electrons during 16 min, as shown in the bottom curve in Figure 8a. In both cases, even the addition of simple olefins, which was considered in our vibrational analysis, does not explain the lack of minimum as their spectra also exhibit minima above the 4.3 eV band. On the other hand, while the difference spectrum, shown as the bottom curve in Figure 8b and obtained from Figure 8a by subtracting the weighted sum containing exposed PA (bottom) from the spectrum of exposed THF (top), still deviates from zero in the 4.7-5.9 eV range (I), the agreement is now much better in the 5.9-6.7 eV range (II). This latter range coincides with the vibronic bands of CO formed in exposed PA. This implies again that CO formed from the aldehyde contributes to the spectrum of exposed THF. In accord with this, the structure that appears at 5.9 to 6.7 eV in THF under exposure [Figure 6a], although barely above the noise level, is reminiscent of the vibronic signature of CO. Furthermore, the residual intensity in range I of the difference spectra demonstrates that additional so far unidentified products stemming from both THF and the aldehyde are present. In an effort to identify the products that are formed together with CO in PA under electron exposure, an increasing percentage of the spectrum of PA [Figure 7a] was subtracted from the


Figure 9. (a) Electronic EEL spectrum of a four-layer film of PA deposited to a six-layer Kr spacer, recorded during 2 min at E0 ) 14 eV after 18 min of irradiation with 14-eV electrons [PAirr, same as Figure 7b]. (b) Same spectrum but after subtraction of the CO spectrum shown in Figure 7c with a weighting factor adjusted until the vibronic structure above 5.9 eV faded away. (c) Difference spectrum obtained by subtracting from the spectrum of exposed PA (PAirr) a percentage of the spectrum of fresh PA [Figure 7a] that was adjusted until the intensity of the 3,1nπ* transitions nearly vanished. (d) Difference spectrum obtained by subtracting further from the result in part b the spectrum of PA with an equal weighting factor as used for curve c. (e) Difference spectrum obtained by subtracting from a spectrum of PA containing 2% of CO (not shown) a certain percentage of the spectrum of pure PA that leads to the disappearance of the PA bands. See text for details of the subtraction procedure.

spectrum of exposed PA [Figure 7b], which is reproduced in Figure 9a, until the intensity of the 3,1nπ* band nearly vanished. The resulting curve, shown in Figure 9c, reveals that the CO bands are superimposed on a steadily rising intensity with an onset at about 4.7 eV. Also, some residual intensity is observed below the 3,1nπ* band threshold at 3.6 eV. Furthermore, the lack of a flat baseline around the 3,1nπ* band indicates that a product contributes also to the intensity in this energy range. It is therefore difficult to determine accurately the contribution of the spectrum of PA to that of exposed PA. Nonetheless, the residual intensity above 4.7 eV cannot be eliminated without generating a strong negative intensity in the range of the 3,1nπ* band. The residual intensity above 4.7 eV can therefore not be ascribed to an artifact of the subtraction procedure but must stem from a product band. The residual intensity between 4.7 and 5.9 eV is reminiscent of similar changes in the spectrum of propene, which have been reported previously and ascribed to the formation of conjugated diene species through oligomerization reactions.18 This interpretation is also consistent with the apparent increase of the baseline (i.e., the spectral range below 3.6 eV) in the spectrum of exposed PA [Figure 9a]. In an effort to visualize the product bands more clearly, the CO bands were further eliminated from the residual intensity of Figure 9c. To achieve this, first an increasing percentage of the spectrum of CO [Figure 7c] was subtracted from the spectrum of exposed PA [Figure 9a] until the CO structure faded away, as shown in Figure 9b. From this

Ja¨ggle et al. curve, the spectrum of PA was further subtracted to the same percentage as the one used in Figure 9c, thus producing the curve shown in Figure 9d. While the latter curve also does not allow us to identify the exact nature of the product, apart from possible contributions of a conjugated diene, it clearly shows that the CO bands are superimposed not only onto the 3ππ* band of PA but also on a broad band resulting from a second product formed upon electron exposure. This finding will have implications on the determination of a cross section for production of CO from PA, as described in the following section. As a last test to support this assignment, we also computed a difference spectrum by subtracting from a spectrum of PA containing 2% of CO a certain percentage of the spectrum of pure PA. In contrast to Figure 9c, a flat baseline was obtained below the onset of the CO bands, as shown in Figure 9e, validating the subtraction procedure and so confirming the formation of a second product in PA under electron exposure. To summarize this section, the electronic spectra give evidence for the direct formation of an aldehyde in THF under electron exposure and indirect production of CO and of a diene from PA as a representative aldehyde. Some residual intensity above 6.7 eV [marked as range III in Figure 8b], which is clearly visible in the difference spectra, could tentatively be explained by contributions from an alcoholic species. For example, the onset of electronic excitations in methanol is located at about 6.7 eV.13,29 This onset does not shift noticeably with increasing alkyl chain length.30 An alcoholic species was also suggested to be formed directly in THF exposed to electrons from the resulting vibrational spectra. 3.3. Determination of Cross Sections for Product Formation. While cross sections for the formation of an aldehyde from THF under electron impact have been measured previously, the aim of the present study is not only to obtain more comprehensive information on the products, but also to investigate to what extent their rate of formation can be quantified. This helps to establish further steps in the reaction sequence taking place in THF under electron exposure. The qualitative analysis of the spectra of THF under exposure to 14- and 15.5-eV electrons has identified as products, among other less defined species, an aldehyde as well as a small amount of CO. In an attempt to elucidate if the aldehyde and CO are formed in consecutive reactions, the cross sections for the formation of aldehyde from THF, of CO from PA as a representative aldehyde, and for the loss of PA were determined separately. Furthermore, an estimate of the amount of CO formed in THF is given. All other possible products do not show a sufficiently clear signature for a quantitative analysis. On the other hand, as the quantification of the formation of aldehyde and of CO suffers from an overlap of the characteristic bands with those of other products, this section also focuses on the development of a procedure to separate the different contributions or, in certain cases, to estimate their relative importance. The determination of the cross section for the formation of aldehyde from THF under electron impact follows the established procedure described previously.13,15,16 Briefly, the formation of products induced by LEE collisions within a sample is described as a single-event process. That is, an electron transfers its energy or part of it to a molecule, which then dissociates or fragments without further reaction with neighboring molecules. The kinetics of the production of a compound P via a reaction A + e- f P + e- is thus described by the following rate law:

dnP I0 ) σPnA dt S0

(1)



Figure 10. Integral intensity in EEL spectra recorded at E0 ) 14 eV during 0.16 min: (0) within the energy loss range 3.6-4.6 eV for the formation of aldehyde in a four-layer film of THF deposited on a sixlayer Kr spacer and (b) within the energy loss range 5.9-6.7 eV for the formation of CO in a four-layer film of PA deposited on a sixlayer Kr spacer, in both cases as a function of displacement of the sample along the X and Y directions after 30 min of exposure to 14-eV electrons. The intensity is given relative to that measured at a nonexposed position and normalized to the maximum value. The obtained triangular shaped profiles indicate a nearly constant electron density over a rectangular area of 0.79 mm by 0.39 mm.

where nA and nP are the number densities of the initially deposited molecules A and of the product P within the film, I0 is the incident current in electrons per second, S0 the irradiated area, and σP is the cross section for the formation of a product P by electron impact at a given incident energy. The current density I0/S0 remains constant throughout the experiment. Similarly, evaluating data only during a time regime where nA remains roughly constant, the cross section can be obtained from

σP )

n P 1 S0 nA t I0

(2)

Here nP/nAt is determined by comparing the signal from a product (i.e., a characteristic EEL feature) obtained after a given exposure time t to those recorded under identical scattering conditions of a series of reference mixtures containing the same or a related compound in different proportions nP/nA. To find the irradiated area S0, a sample is exposed to the electron beam so as to make a well-defined imprint of the products. After the exposure, the signal from a product is monitored upon translating the sample along the X and Y directions, thus generating two intensity profiles. The intensity profiles for the aldehyde and CO signals, generated by exposing respectively four-layer films of THF and PA condensed on a six-layer Kr spacer to 14-eV electrons during 30 min, are presented normalized to their respective maximum in Figure 10. Each point corresponds to the intensity integrated within a specific energy-loss range in an EEL spectrum recorded at E0 ) 14 eV during 0.16 min, in the 3.6-4.6 eV range for the formation of aldehyde in THF, and in the 5.9-6.7 eV range

Figure 11. Integral intensity between 3.6 and 4.6 eV (i.e., in the range of the 3,1nπ* transitions of aldehyde as indicated in Figure 6a) in EEL spectra recorded at E0 ) 14 eV during 2 min, I(3,1nπ*), for a fourlayer film of THF deposited on a six-layer Kr spacer as a function of exposure time to 14-eV electrons (9) and for four-layer films of THF: PA mixtures deposited on a six-layer Kr spacer and containing an increasing percentage of PA (O), as given on the top scale. The solid line represents a linear fit to the data of the exposure experiment during the initial 10 min.

for the formation of CO in PA. The profiles exhibit a triangular shape with a fwhm of (0.79 ( 0.10) and (0.39 ( 0.05) mm in the X and Y directions, respectively. This result indicates that the distribution of products was practically uniform over a rectangular area of S0 ) (3.1 ( 0.6) × 10-3 cm2, the latter being essentially delimited by the electrostatic image of the rectangular exit slit of the monochromator at the sample. To quantify the production of aldehyde from THF under electron exposure, the signal from the 3,1nπ* band of aldehyde in Figure 6a was compared to that of reference mixtures containing different percentages of PA. The latter compound was used as a model aldehyde on the basis that the 3,1nπ* band is similar among the different aldehydes in Figure 6b. The aldehyde signal produced by 14-eV electrons incident on a fourlayer film of THF deposited on a six-layer spacer of Kr is shown as a function of electron exposure time in Figure 11. Each of these points was obtained from the integral intensity of the 3,1nπ* band between 3.6 and 4.6 eV in the EEL spectrum recorded at E0 ) 14 eV during 2 min, as shown by the vertical dashed lines in Figure 6a. This signal, which is ascribed mainly to the aldehyde, increases linearly during at least the first 10 min. Comparing this signal increase to that obtained from equally thick films of reference mixtures deposited on a six-layer spacer of Kr and containing increasing percentages of PA, shown as open symbols in Figure 11, yields an initial aldehyde production rate of (0.026 ( 0.002)%/s. Given the incident electron current I0 ) (1.2 ( 0.2) nA and the irradiated area S0 as deduced above, we find a cross section of σPA ) (1.0 ( 0.4) × 10-16 cm2 for aldehyde production from THF at E0 ) 14 eV. The cross section derived from this procedure nonetheless represents an upper bound to the true cross section owing to overlap with the 3ππ* band of olefins [Figure 6c] that are also formed in THF according to the vibrational spectra. The CO bands in the spectra of the mixture and in the spectra of PA under electron exposure [Figure 7b] are superimposed onto overlapping contributions of the 3ππ* band of PA and those due to other products that could not be identified unequivocally. Therefore, additional steps were needed as compared to the above procedure to allow a close comparison between the


Ja¨ggle et al.

Figure 12. Ratio of the integral intensities between 5.9 and 6.7 eV, I(CO), and between 3.6 and 4.6 eV, I(3,1nπ*) (i.e., in the ranges of the a3Π state of CO and of the 3,1nπ* transitions of PA as indicated in Figure 7), for a four-layer film of PA deposited on a six-layer Kr spacer as a function of exposure time to 14-eV electrons (9) and for fourlayer films of PA:CO mixtures deposited on a six-layer Kr spacer and containing an increasing percentage of CO, as given on the top scale (O). The solid line represents a linear fit to the 20 min data of the exposure experiment.

integral intensity of the CO bands in the spectra of exposed PA and that of the reference mixtures containing CO. In the case of the reference mixtures, the spectrum of PA [Figure 7a] was first subtracted until a flat baseline was obtained below 5.9 eV, as shown for one specific composition in Figure 9e, and then the integration was performed between 5.9 and 6.7 eV. In the case of exposed PA [Figure 9a], the curve produced by subtraction of CO, as exemplified by Figure 9b, was used as a baseline in the integration procedure. The CO signal was thus obtained from the intensity integrated between 5.9 and 6.7 eV in Figure 9a minus that within the same range in Figure 9b. Unfortunately, the absolute intensity fluctuated slightly among the reference spectra due to unknown reasons. Taking advantage of the presence of the 3,1nπ* band from aldehydes in the spectra of the reference mixtures and exposed PA, the CO signal obtained in each case was further normalized to the integral of the 3,1nπ* band. Figure 12 shows this normalized signal as a function of exposure time for a four-layer film of PA deposited on a six-layer Kr spacer along with that from equally thick films of reference mixtures. The comparison between these two data sets gives an initial CO production rate of (0.0021 ( 0.0007)%/ s. Using the same incident current and irradiated area as in the THF case, we find a cross section of σCO ) (8.7 ( 3.0) × 10-18 cm2 for CO production from PA at E0 ) 14 eV. This value represents a lower bound to the cross section as again the 3ππ* band of some olefin produced under electron exposure contributes to the intensity of the 3,1nπ* band of PA. The cross section for the loss of PA under exposure to 14eV electrons, σPA,loss, was obtained directly from the intensity decrease of the 3,1nπ* transitions for a four-layer film of PA deposited on a six-layer Kr spacer as a function of exposure time and applying the following kinetic equation:

nPA I0 ln 0 ) -σPA,loss t S0 nPA

(3)

In this expression, nPA0 is the number density of PA initially in the film (i.e., t ) 0) and nPA that after a given exposure time t.

Figure 13. Logarithmn of the baseline corrected integral intensity between 3.6 and 4.6 eV, I(3,1nπ*) (i.e., in the range of the 3,1nπ* transitions of PA as indicated in Figure 7), normalized to its initial value I0(3,1nπ*) for a four-layer film of PA deposited on a six-layer Kr spacer as a function of the time of exposure to 14-eV electrons. The solid line represents the linear fit to these data during the first 12 min of exposure.

This analysis again must take into account that some olefins and conjugated dienes are formed under electron exposure. Olefins would lead to new intensity in the range of the 3,1nπ* band, whereas conjugated dienes lead to the observed apparent increase of the background. Comparison with the EEL spectrum of butadiene31 suggests that the diene signal within the range of the 3,1nπ* band accounts for an average intensity corresponding to the height of the 3,1nπ* band onset. Consequently, this contribution was roughly removed prior to the integration of the 3,1nπ* band between 3.6 and 4.6 eV by subtracting a constant intensity adjusted to the apparent baseline below 3.6 eV. Figure 13 shows the logarithmic plot of the resulting integral intensity, I(3,1nπ*), normalized to the initial value, I0(3,1nπ*), which was obtained during the first 2 min, versus the exposure time. By using the same incident current and irradiated area as above, eq 3 yields a cross section of σPA,loss ) (2.3 ( 0.9) × 10-16 cm2 for the loss of PA at E0 ) 14 eV. Nonetheless, owing to possible overlap with an olefin 3ππ* band, this value represents a lower bound to the cross section. Finally, a crude estimate of the production of CO following exposure of THF to electrons was obtained by visual inspection of the spectra of reference mixtures of THF and CO. Figure 14 shows such spectra obtained for four-layer reference mixture films deposited on six-layer Kr spacers and recorded at E0 ) 14 eV. The vibronic structure of CO becomes faintly visible at a CO content of 2% and clearly discernible at 4%. Given that some weak hint of the CO bands is seen in the spectrum of THF after 20 min of exposure suggests that between 1% and 2% of CO must be present. The implications of this observation on the kinetics of electron-induced reactions in THF will be discussed in the following section. 3.4. Kinetic Model of the Reaction Sequence and Discussion of Cross Sections. As the quantity of CO formed in THF under electron exposure can only be roughly estimated, it is not easy to decide whether all of the CO formed in the reaction mixture is in fact a secondary product, i.e., stems from a reaction of the primary aldehyde product. To obtain a better estimate, a kinetic model based on the cross sections determined in the previous section is formulated and evaluated here. Assuming as a starting point that exposure of THF to electrons produces only the aldehyde with an upper bound to the cross section of σPA ) (1.0 ( 0.4) × 10-16 cm2, that CO is only


J. Phys. Chem. B, Vol. 110, No. 25, 2006 12521 an aldehyde requires breaking of a C-H bond, whereas in acetone, only C-C bonds need to be broken. From standard thermodynamic data, it is estimated that the latter process requires less energy than the former. Therefore releasing CO from an aldehyde may be less favorable than releasing CO from a ketone. 4. Conclusions

Figure 14. Electronic EEL spectra recorded at E0 ) 14 eV on fourlayer mixture films containing an increasing percentage of CO in THF deposited on a six-layer Kr spacer. Vertical dashed lines show the positions of the first three CO bands.

formed from PA with σCO ) (8.7 ( 3.0) × 10-18 cm2, and that PA is decomposed with a cross section of at least σPA,loss ) (2.3 ( 0.9) × 10-16 cm2, the following coupled kinetic equations,

dnTHF I0 ) -σPAnTHF dt S0 I0 dnPA ) (σPAnTHF - σPA,lossnPA) dt S0 dnCO I0 ) σCOnPA dt S0

(4)

predict that between 0.1% and 0.5% CO would be formed within the reaction mixture during 20 min of exposure. According to Figure 14, this is not enough to explain the faintly visible structure of CO in the electronic spectrum of THF after this same exposure time [Figure 6a]. Assuming that the cross section for CO formation is roughly the same among the different aldehydes, this finding strongly suggests that another reaction pathway, which does not involve an aldehyde, exists for the production of CO. Adding such a reaction for the direct production of CO from THF along with the corresponding one for the loss of THF to the kinetic model [eq 4], and varying the cross section for this process, shows that this latter must have a value of between 2 × 10-18 and 7 × 10-18 cm2 to explain the observed amount of CO. The nature of such a reaction is probably complex as massive bond reorganization is required to release CO from THF without intermediate formation of an aldehyde. On the other hand, the biradical proposed above as the intermediate product of THF would be a natural precursor of CO. Similarly, the formation of CO from methanol, which requires a similarly massive bond reorganization, has been observed with a comparable cross section of about 4 × 10-18 cm2 at and near 14 eV.13 Therefore, the present estimated cross section for the direct production of CO from THF does not seem unreasonable. On the other hand, the obtained cross section for the formation of CO from PA appears rather small compared to the previously determined value of 6 × 10-17 cm2 for CO production from acetone at 14 eV.16 This may nonetheless be rationalized by the stability of the resulting radical species. Release of CO from

Exposing a multilayer film of THF to electrons of energies around 15 eV was found to follow mainly a reaction sequence where THF produces an aldehyde, and then the aldehyde produces CO. It was deduced from further analysis of the vibrational and electronic EEL spectra of such exposed films that a number of other products such as simple and conjugated olefins, and possibly alcoholic species, were also formed. Because of this and of the superposition of the bands from different products within the EEL spectra, the cross sections for the formation of some specific products were difficult to determine exactly. Therefore, upper and lower bounds for these cross sections were obtained. These values were used in a kinetic model showing that additional reaction pathways are most probably involved and also lead to the formation of CO. In retrospect, the present results illustrate the complexity of electron-induced reactions even in the case of the apparently simple THF molecule. Although the high sensitivity of EEL spectroscopy allowed us to measure the cross sections, because of the restricted resolution, great care has to be applied concerning their interpretation. Nonetheless, spectral summations and subtractions are possible to some extent and give valuable information about the reactions occurring in solid molecular samples under the effect of exposure to low-energy electrons. Acknowledgment. This work was sponsored by the Canadian Institutes of Health Research and the DFG under grant Sw26/10-1. References and Notes (1) Di, W.; Rowntree, P.; Sanche, L. Phys. ReV. B 1995, 52, 16618. (2) Pylant, E. D.; Hubbard, M. J.; White, J. M. J. Phys. Chem. 1996, 100, 15890. (3) Eck, W.; Stadler, V.; Geyer, W.; Zharnikov, M.; Go¨lzha¨user, A.; Grunze, M. AdV. Mater. 2000, 12, 805. (4) La, Y.-H.; Kim, H. J.; Maeng, I. S.; Jung, Y. J.; Park, J. W. Langmuir 2002, 18, 301. (5) Balog, R.; Illenberger, E. Phys. ReV. Lett. 2003, 91, 213201. (6) Boudaiffa, B.; Cloutier, P.; Hunting, D.; Huels, M. A.; Sanche, L. Science 2000, 287, 1658. (7) Antic, D.; Parenteau, L.; Lepage, M.; Sanche, L. J. Phys. Chem. B 1999, 103, 6611. (8) Lu, Q.-B.; Sanche, L. J. Chem. Phys. 2004, 120, 2434. (9) Cobut, V.; Frongillo, Y.; Patau, J. P.; Goulet, T.; Fraser, M.-J.; Jay-Gerin, J.-P. Radiat. Phys. Chem. 1998, 51, 229. (10) Bass, A. D.; Sanche, L. Radiat. EnViron. Biophys. 1998, 37, 243. (11) Tai, Y.; Shaporenko, A.; Eck, W.; Grunze, M.; Zharnikov, M. Langmuir 2004, 20, 7166. (12) Martel, R.; McBreen, P. H. J. Chem. Phys. 1997, 107, 8619. (13) Lepage, M.; Michaud, M.; Sanche, L. J. Chem. Phys. 1997, 107, 3478. (14) Winterling, H.; Haberkern, H.; Swiderek, P. Phys. Chem. Chem. Phys. 2001, 3, 4592. (15) Breton, S.-P.; Michaud, M.; Ja¨ggle, C.; Swiderek, P.; Sanche, L. J. Chem. Phys. 2004, 121, 11240. (16) Lepage, M.; Michaud, M.; Sanche, L. J. Chem. Phys. 2000, 113, 3602. (17) Swiderek, P.; Deschamps, M. C.; Michaud, M.; Sanche, L. J. Phys. Chem. B 2004, 108, 11850. (18) Swiderek, P.; Deschamps, M. C.; Michaud, M.; Sanche, L. J. Phys. Chem. B 2003, 107, 563. (19) Ibach, H. Electron Energy Loss Spectrometers; Springer: New York, 1991). (20) Swiderek, P.; Winterling, H. Chem. Phys. 1998, 229, 295. (21) Sanche, L.; Michaud, M. Phys. ReV. B 1984, 30, 6078.

12522 J. Phys. Chem. B, Vol. 110, No. 25, 2006 (22) Michaud, M.; Cloutier, P.; Sanche, L. ReV. Sci. Instrum. 1995, 66, 2661. (23) Sanche, L. J. Chem. Phys. 1979, 71, 4860. (24) Lepage, M.; Letarte, S.; Michaud, M.; Motte-Tollet, F.; HubinFranskin, M.-J.; Roy, D.; Sanche, L. J. Chem. Phys. 1998, 109, 5980. (25) NIST webbook: http://webbook.nist.gov/chemistry/. (26) Zhao, H.; Kim, J.; Koel, B. E. Surf. Sci. 2003, 538, 147. (27) Michaud, M.; Sanche, L. Unpublished result.

Ja¨ggle et al. (28) Taylor, S.; Wilden, D. G.; Comer, J. Chem. Phys. 1982, 70, 291. (29) Michaud, M.; Fraser, M. J.; Sanche, L. J. Chim. Phys. 1994, 91, 1223. (30) Robin, M. B. Higher Excited States of Polyatomic Molecules; Academic Press: New York, 1975; Vol. II. (31) Swiderek, P.; Michaud, M.; Sanche, L. J. Chem. Phys. 1993, 98, 8397.

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