J. Phys. Chem. C 2007, 111, 303-311
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Fate of Reactive Intermediates Formed in Acetaldehyde under Exposure to Low-Energy Electrons P. Swiderek,* C. Ja1 ggle, D. Bankmann,† and E. Burean Institute for Applied and Physical Chemistry, UniVersita¨t Bremen, Fachbereich 2 (Chemie/Biologie), Leobener Strasse/NW 2, Postfach 330440, 28334 Bremen, Germany ReceiVed: August 22, 2006; In Final Form: October 12, 2006
Chemical reactions have been induced in condensed acetaldehyde by exposure to low-energy electrons as demonstrated by use of high-resolution electron energy loss (HREEL) spectroscopy and thermal desorption spectroscopy (TDS). The HREEL spectra show that the acetaldehyde film is modified significantly when the incident electron energy is increased above 10 eV. Release of CO upon exposure to electrons at an incident energy of 14 eV is observed by electronic HREEL spectra. The changes in the vibrational spectra upon exposure of the molecular film to electrons at an incident energy of 15.5 eV are analyzed in detail using reference spectra for a number of potential product molecules. TDS data reveal clearly the decomposition of acetaldehyde and formation of CO and CH4 as major products under electron exposure. The combined results also give evidence for the formation of propionaldehyde. Other less prominent products are acetone and possibly an alcoholic species. A reaction mechanism is suggested that rationalizes the formation of larger products. It involves fragmentation of the molecule releasing CO, abstraction of an H atom from an adjacent acetaldehyde by the H radical fragment, and, finally, recombination of the resulting radical with the remaining CH3 fragment.
1. Introduction Reactions induced by exposure of condensed matter to lowenergy electrons are important in different fields of technology such as resist patterning by electron beam lithography1 or radiation damage in biological matter like the DNA.2 In most applications, high-energy radiation is used. The actual chemical reaction, however, is often induced by secondary electrons produced under the effect of high-energy radiation.3 To get an insight in the mechanism of those reactions, a comprehensive understanding of the reaction sequences is required. With the choice of acetone and methanol as examples, studies on electron-induced reactions in the condensed phase using highresolution electron-energy-loss (HREEL) spectroscopy have initially concentrated on the formation of CO upon fragmentation of the samples with specific focus on the determination of absolute cross sections for this process.4,5 Also, formation of the isomerization product propene was observed and quantified for condensed cyclopropane6,7 Recently, the production of an aldehyde in condensed tetrahydrofuran (THF) was investigated as another example of a more complex product molecule.8 Nonetheless, to understand more comprehensively the reactions that occur, other products must be identified as well. Therefore, reaction sequences in THF were also studied.9 This investigation aimed not only at obtaining more information on the immediate products formed upon electron exposure but also on reactions of propionaldehyde as model for the aldehyde formed from THF in the initial reaction step, which, in turn, yields CO. In acetaldehyde, which represents an even simpler model, production of CO is expected to be accompanied by release of reactive * Author to whom correspondence should be addressed. † Present address: Institut fu ¨ r Organische Chemie, Universita¨t zu Ko¨ln, Greinstr. 4, 50939 Ko¨ln, Germany.
CH3 or H fragments, disregarding for now the charge state of the products:
CH3CHO f CH3 + CO + H
(1)
It is very likely that at low-incident electron energies (E0) the initial step of electron-induced reactions proceeds via a temporary negative ion state formed by attachment of the electron.4,5 For gaseous acetaldehyde such a process ascribed to Feshbach resonances has been identified from the production of CH3- upon interaction with electrons in the 6-7 eV range.10 At higher energies as applied in this study, ionization processes become accessible and yield different fragments, including CH3, CO, or their charged counterparts and H.11 Again, on the mere basis of stochiometry and disregarding the charge, CH3 and H fragments may recombine to form methane
CH3 + H f CH4
(2)
while recombination of two CH3 fragments would yield ethane:
2CH3 f C2H6
(3)
In addition, in the condensed phase, reactions of the initial reactive fragments with remaining acetaldehyde leading to larger products are anticipated on the basis of earlier findings showing that oligomerization reactions may occur under exposure to lowenergy electrons.12 This work aims at identifying products due to reactive intermediates that are formed in the initial step of a reaction induced by low-energy electrons at E0 ) 15.5 eV in condensed acetaldehyde. Production of CO has been clearly observed at this energy in all of the oxygen containing molecules that have been investigated so far.4,5,8,9 In this work, HREEL spectroscopy
10.1021/jp065412u CCC: $37.00 © 2007 American Chemical Society Published on Web 11/29/2006
304 J. Phys. Chem. C, Vol. 111, No. 1, 2007 in the range of electronic excitations is used to show that CO is also formed in condensed acetaldehyde. HREEL spectroscopy in the range of the vibrational excitation and thermal desorption spectroscopy (TDS) are performed to monitor the reactions occurring upon exposure to low-energy electrons. The results give evidence that not only CO and CH4 but also larger reaction products are formed. A mechanism that rationalizes this finding is proposed and discussed. 2. Experimental Section The HREEL experiments were performed with a spectrometer with cylindrical deflectors13 incorporated in a µ-metal-shielded UHV chamber pumped to a base pressure of 1 × 10-10 Torr by the combined action of an ion pump and a titanium sublimation pump. This setup has been described in detail elsewhere.14 The monochromator, which can be rotated between 14 and 120° with respect to the normal of the sample, was set to 60° for the present experiments. The analyzer is fixed at 60° at the opposite azimuth. The resolution was adjusted between 10 and 15 meV full width at half-maximum (fwhm) for transmitted currents on the polycrystalline Pt substrate ranging from 0.1 to 0.2 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.14 The TDS experiments were performed in an ultrahigh vacuum (UHV) chamber pumped by a turbomolecular pump to a pressure of about 10-10 Torr with the residual gas consisting mainly of hydrogen. If necessary, additional pumping can be provided by a titanium sublimator. Low-energy electron-induced reactions in acetaldehyde condensed at 35 K onto a polycrystalline Au sheet were monitored by thermal desorption spectrometry (TDS) using a quadrupole mass spectrometer (QMS) residual gas analyzer (Stanford, 200 amu) with electron impact ionization at 70 eV. The temperature was measured using a thermocouple type E press-fitted to the Au substrate. A commercial fload gun delivering currents of the order of a few µA/cm2 as measured at the substrate was used for electron exposure. The energy (E0) of the non-monochromatized electron beam with an estimated resolution of the order of 0.5-1 eV was set to 15 eV. All experiments were performed on thin molecular films deposited on the metallic substrates held at cryogenic temperatures by a closed-cycle helium refrigerator (Leybold Vacuum). To produce these films, 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 metal substrate. Prior to each deposition the HREEL substrate is cleaned by direct resistive heating to an orange glow and the thicker TDS substrate by resistive heating of two thin Ta ribbons spotwelded to the thicker Au whereby the substrate reaches 273 K. In principle, the substrate used for TDS can be heated to higher temperatures by more intense resistive heating of the Ta ribbons. In the present experiments we were only concerned with processes in the physisorbed multilayers. The material evaporated efficiently well below 273 K while probably leaving behind some material, including a minor amount of products with large molecular mass or CO chemisorbed to more reactive sites of the Au substrate, which we assume to be passivated in consequence. Therefore, the temperature of 273 K was considered sufficient to reproducibly create a nonreactive surface free of material that would contribute to the next TDS scan.
Swiderek et al. Acetaldehyde (AA, stated purity of 99.95%) and hexane (C6, spectroscopic grade) were purchased from Fluka; methane (C1, 99.995%), ethane (C2, 99.95%), ethylene (ET, 99.95%), and propene (PR, 99.98%) were from Messer Griesheim. Propionaldehyde (PA, 99 + %) was obtained from Acros Organics, and butyraldehyde (BA, >99%), from Merck. For the measurements of electronic spectra in Sherbrooke acetaldehyde was purchased from Aldrich Chemical Ltd. (99.5+%). The provenience and purity of methanol (ME) and acetone (AC) used for recording the HREEL spectra was described in the original publications.15,16 Acetone (99.98%), 2-propanol (99.5%), and 1-propanol (99.8%) used in TDS experiments were purchased from Fisher Scientific, Janssen Chimica, and Fluka, respectively. 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 in HREEL experiments was estimated by comparing the amount of gas with the previous monolayer calibrations for cyclopropane6 and benzene14 taking into account the differences in molecular size. The film thickness in the TDS experiments was estimated by recording TDS curves of acetaldehyde for increasing coverage. Acetaldehyde desorption shows a weak but characteristic peak that rapidly saturates and is ascribed to the first layer on the Au and a second peak at slightly lower temperature that starts to increase upon saturation of the monolayer peak. The thickness was again chosen well in the multilayer regime, i.e., between 20 and 30 layers. Changes within the molecular films upon exposure to lowenergy electrons were monitored both by HREEL spectroscopy and TDS. In HREEL experiments, series of spectra are continuously recorded on the same sample as a function of exposure time. Any observable reaction in the course of such an exposure experiment thus takes place within the area of the surface probed by the electron beam of the spectrometer. At the chosen film thickness of 5-10 monolayers, the intensities within the vibrational spectra have reached their maximum value. This indicates that only the topmost layers of the films are effectively probed. Interactions with the underlying substrate, therefore, have a negligible effect on the HREEL results presented here. Furthermore, due to the low deposition temperature the molecular films are largely disordered. Changes in the intensities during an exposure experiment can therefore not be ascribed to changes in the diffraction properties of the film.17,18 While most HREEL spectra have been acquired in the range of vibrational excitations, CO production was probed similarly to previous studies4,5 by recording series of electronic HREEL spectra, both using the instrument described above and a different setup located in Sherbrooke, Canada, that has been described in detail earlier.4,5,7-9,19 This latter instrument, which was run at a typical resolution of 28 meV, has the capability of cooling the single-crystal Pt(111) substrate to around 25 K which is sufficient to condense CO while the lowest temperature reading achieved on the Bremen setup during the time of the present experiments was nominally 35 K. While the currents impinging on the sample were of similar order of magnitude in the Bremen and Sherbrooke instruments, the irradiated area in the former is much larger. The size of the beam in the Bremen instrument has been estimated by displacing the sample with respect to the spectrometer and observing the drop in current as the beam reaches the edge of the sample. Because of the larger area, exposure times on the Bremen instrument are long compared to the experiments performed with the Sherbrooke setup. Nonetheless, the longer recording times together with the
Acetaldehyde under Exposure to Low-Energy Electrons
Figure 1. Top: Electronic HREEL spectrum recorded during 2 min at E0 ) 14 eV for a multilayer film of acetaldehyde freshly deposited onto a 6-layer film of Kr on Pt(111) (AA) and for the same film after 9 min of exposure (AAexp). Bottom: Electronic HREEL spectrum recorded at E0 ) 15.5 eV for a multilayer film of acetaldehyde immediately after deposition on Pt (AA) and after 1 night of exposure at the same E0 (AAexp). For comparison, a spectrum of the lowest electronic transition of CO4,5 is included.
higher resolution allow us to resolve clearly the progress of changes in the spectra occurring under exposure as each spectrum can be recorded with a satisfactory signal-to-noise ratio. In TDS experiments, changes of the intensity of specific mass peaks between the nonexposed and the exposed sample yield information on the decomposition of the initial sample and the formation of products upon electron exposure and subsequent temperature increase. In an exposure experiment, thermal desorption was induced after an irradiation time corresponding to an electron exposure of 2000 µC by increasing the substrate temperature from 35 K up to 270 K at a rate of 1 K/s using a home-built temperature control and data acquisition unit. In each experiment, TDS curves were recorded for four different molecular masses, among which in all cases are the masses of the parent positive ions of the investigated molecules, which are formed at the entrance of the QMS. The other chosen signals were characteristic of specific products as also discussed later for the larger investigated molecules. As a reference, a TDS measurement was performed using a nonexposed film of the same composition and thickness prior to each exposure experiment (denoted as 0 µC). 3. Results 3.1. HREEL Spectroscopy. Figure 1 shows the electronic spectrum of acetaldehyde recorded for a freshly deposited film and for a film after a certain time of exposure as recorded on two different HREEL setups, each at the lowest obtainable temperature. After 9 min of exposure the spectrum of a film deposited at a temperature of 25 K in the Sherbrooke setup and recorded at E0 ) 14 eV shows the signature of CO as obvious from the comparison with the spectrum of pure CO,4,5 which is also included in Figure 1. On the contrary, the analogous exposure experiment performed at a temperature reading of 35 K in the Bremen setup during one night at E0 ) 15.5 eV does not show evidence of CO. On the basis of the previously published cross sections for other examples,4,5 it is unlikely that
J. Phys. Chem. C, Vol. 111, No. 1, 2007 305
Figure 2. Vibrational HREEL spectra recorded during 53 min at different E0 values for freshly prepared multilayer acetaldehyde films deposited on Pt and for the same films after the stated prolonged exposure to electrons at the same E0.
the reaction rate should be drastically different for the two energies. Considering also the results described in the following, we rather conclude that CO evaporated from the condensed film in the Bremen setup. Figure 2 shows the vibrational HREEL spectra of acetaldehyde before and after prolonged exposure to electrons at the given energy for four different E0. All spectra show bands at 63, 95, 109, 137, 175, 212, and 368 meV. The assignments are given according to previous data from optical spectroscopy11,20 and take into account that very close vibrational bands are not resolved in the present spectra. Additional small peaks occur because of multiple inelastic scattering within the molecular film. The changes within the spectra upon exposure are very weak at E0 ) 8.5 eV but become more pronounced with increasing E0. More specifically, at 15.5 eV, the intensity in the range of the CCO deformation vibration (ν10) and of the CO stretching vibration (ν4) decreases considerably. The latter band also broadens at the low-energy side. A weak drop in intensity is observed for the other bands except around 100 meV whereas the intensity clearly increases in the minima between the three lowest bands and somewhat between the 137 and 175 meV peaks. Similar but weaker changes are observed at E0 ) 10.5 eV and E0 ) 12.5 eV. At the typical resolution of the HREEL spectrometer used here, a loss of intensity in the vibrational spectrum does not necessarily correlate to a loss of the initial compound. Rather, the overlap with the spectra of the reaction products must be considered. Therefore, reference spectra were recorded for a number of different molecules (Figures 3 and 4) that may be expected to be formed under electron exposure. The spectra of fresh and exposed acetaldehyde are included in these figures for comparison. In the simplest case, methane or ethane could be produced through reactions 2 and 3. Ethylene (C2H4) and propene (CH3CHCH26) are included as simple model compounds representing unsaturated products. Hexane (C6H14) serves as a model of a saturated oligomerization product. Hydrogenation of the CO double bond in acetaldehyde would lead to an alcohol. Therefore, the spectrum of methanol (CH3OH15) is also included as reference. Longer aldehydes, represented here by propional-
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Figure 3. Vibrational HREEL spectra of multilayer films of methane (C1), ethane (C2), hexane (C6), and fresh and exposed ethylene (ET, ETexp) recorded at E0 ) 15.5 eV and of fresh and exposed propene (PR, PRexp) recorded at 14.7 eV.6 Exposure in each case took place at the same E0 as recording of the spectrum. For comparison, the vibrational HREEL spectra of a freshly prepared multilayer acetaldehyde film deposited on Pt recorded at E0 ) 15.5 eV (AA) and of the same film after the stated prolonged exposure to electrons at the same E0 (AAexp) from Figure 2 are also included.
Figure 4. Vibrational HREEL spectra of multilayer films of methanol (ME) recorded at 16.4 eV,13 of fresh and exposed propionaldehyde (PA, PAexp) and fresh and exposed butyraldehyde (BA, BAexp) recorded at E0 ) 15.5 eV, and of acetone (AC) recorded at 16.0 eV.14 Exposure in each case took place at the same E0 as recording of the spectrum. For comparison, the vibrational HREEL spectra of a freshly prepared multilayer acetaldehyde film deposited on Pt recorded at E0 ) 15.5 eV (AA) and of the same film after the stated prolonged exposure to electrons at the same E0 (AAexp) from Figure 2 are also included.
dehyde (CH3CH2CHO) and butyraldehyde (CH3CH2CH2CHO), are other potential oligomerization products, of which the former
Swiderek et al.
Figure 5. Vibrational HREEL spectra of a freshly prepared multilayer acetaldehyde film deposited on Pt recorded at E0 ) 15.5 eV (AA) and of the same film after a 265 min exposure to electrons at the same E0 (AAexp) and attempted reproductions of the latter by weighted sums of the spectra of fresh acetaldehyde and the spectra from Figure 3 of methane (AA + C1), exposed ethane (AA + C2exp), hexane (AA + C6), exposed ethylene (AA + ETexp), and propene (AA + PR). To facilitate the comparison, the thin lines plotted together with the weighted sums reproduce the spectrum AAexp.
may occur given that the methyl group of an intact acetaldehyde is attacked by a hydrocarbon radical. Finally, acetone (CH3COCH316) could be produced if a CH3 fragment attaches to the carbon of the aldehyde group. The assignments of the spectra6,11,15,16 are not discussed in detail here. A complete assignment of the spectra of the longer aldehydes is not available to the best of our knowledge. We identify the most important features above 150 meV as related to those of acetaldehyde while characteristic bands due to different deformation and C-C stretching vibrations appear in the lower-energy part of the spectrum. Figure 3 shows that formation of alkanes alone cannot explain the changes occurring in the HREEL spectrum of acetaldehyde under exposure to the electron beam because bands that could explain the observed intensity increase around 80 meV are absent in the alkane spectra. On the other hand, the spectra of propene and the aldehydes with longer carbon chain do exhibit bands at or near this energy. Still, the contribution of the different potential products is difficult to judge from the mere comparison of the spectra. Therefore, following an earlier approach,6 an effort was undertaken to reproduce the spectra of acetaldehyde after exposure by a weighted sum of the spectra of fresh acetaldehyde and of the different reference spectra included in Figures 3 and 4. To facilitate this analysis and to get an insight into the early stages of the reaction, the reproduction was attempted for acetaldehyde after moderate exposure (265 min, Figures 5 and 6). Comparing with earlier findings for cyclopropane6,7 and taking into account that the magnitude of the absolute reaction cross sections is similar for cyclopropane7 and propionaldehyde,9 the latter being closely related to acetaldehyde, we find it might be expected that such
Acetaldehyde under Exposure to Low-Energy Electrons
Figure 6. Vibrational HREEL spectra of a freshly prepared multilayer acetaldehyde film deposited on Pt recorded at E0 ) 15.5 eV (AA) and of the same film after a 265 min exposure to electrons at the same E0 (AAexp) and attempted reproductions of the latter by weighted sums of the spectra from Figure 4 of fresh acetaldehyde and the spectra of methanol (AA + ME), fresh and exposed propionaldehyde (AA + PA, AA + PAexp), fresh and exposed butyraldehyde (AA + BA, AA + BAexp), and acetone (AA + AC). To facilitate the comparison, the thin lines plotted together with the weighted sums reproduce the spectrum AAexp
an exposure time on the Bremen HREEL instrument leads to only minor contributions from secondary reactions. Nonetheless, at least the different aldehydes should show similar reactivity upon exposure to low-energy electrons. Similarly, it was shown previously for the case of cyclopropane that products of such secondary reactions contribute to the spectra of the sample after prolonged exposure.6 While spectra of the reaction mixture at low exposure in this case could be reproduced by a weighted sum of the spectra of fresh cyclopropane and fresh propene, which is the dominant initial product, spectra acquired after prolonged exposure were better approximated by using a spectrum of propene after prolonged exposure instead of the fresh sample. Thus, not only reference spectra of freshly deposited films but also of films after prolonged exposure are included in Figures 3 and 4 and used for further analysis. As only a qualitative analysis is performed using the procedure described above, the exposure times are not explicitely stated for the reference spectra. Spectra that are not denoted as exposed (exp) have been obtained by using data acquired during a time interval in which changes to the spectrum were not yet visible. Exposure times for spectra denoted as exposed are of similar order of magnitude as for acetaldehyde. This approach allows us at least to judge whether the agreement between the weighted sum and the spectrum of exposed acetaldehyde can be improved by considering secondary reactions. Another approximation consists in the use of spectra from literature acquired at a slightly different E0 for propene,6 methanol,15 and acetone.16 This is justified for a qualitative analysis because inspection of the original data15,16 shows that the relative spectral
J. Phys. Chem. C, Vol. 111, No. 1, 2007 307 intensities within these spectra show only marginal changes with E0 within the range spanned by the data for Figures 3 and 4. Figures 5 and 6 show the best fit to the spectrum of exposed acetaldehyde that could be obtained by adjusting the weighting factor in the sum of the spectra of acetaldehyde and the reference compound. The weighting factors of the two spectra contained in the weighted sums do not represent meaningful values because the absolute intensities of the single spectra have not been calibrated. This would require the comparison with the spectra of mixtures in which the composition is known. Therefore, the factors are not stated here. Instead, a qualitative comparison of the relative intensities between the weighted sum and the spectrum of exposed acetaldehyde is performed. To this end, all spectra shown in Figures 5 and 6 are normalized to the height of the band at 175 meV. To facilitate the comparison, the single curves have been smoothed slightly by performing an averaging over adjacent points. The spectrum of acetaldehyde after moderate exposure (Figures 5 and 6) already reveals the same characteristic changes that were described above for longer exposure time, although these changes are less pronounced. An attempt to reproduce the spectrum after exposure by a weighted sum with the spectra of the different alkanes fails as expected (Figure 5). The weighted sum containing the spectrum of fresh ethane is not shown because the deviations were found to be always even larger than for exposed ethane. Nonetheless, for all three alkanes and besides other smaller deviations, the minima at 80 and 120 meV stay much too pronounced and the peak at 139 meV is too weak in the weighted sum. According to reaction 2, methane would be a very likely side product of CO formation. The next section will demonstrate that this is in fact the case. Therefore, the finding that methane does not explain the changes in the spectra of acetaldehyde suggests that, similar to CO, methane produced under electron exposure actually has evaporated due to insufficient cooling capacity at the time of the experiment. Together with the TDS results discussed in the next section, this also shows that the temperature calibration in the HREEL experiment is not accurate but that the stated temperature only serves as an approximate value. Construction of the weighted sum using the spectra of ethylene or propene instead of those of alkanes leads to a better overall reproduction, especially in the range of the low-energy minima, of the spectrum of acetaldehyde after moderate exposure (Figure 5) with only minor differences between reference spectra of fresh and exposed samples. Still, the bands at 63 and 211 meV are slightly too intense, while the peak at 139 meV is slightly too weak. In the case of propene the sloping of the 100 meV band toward low losses is reproduced correctly for the first time. Nonetheless, the difference between adjacent minima and maxima below 100 meV remains overestimated. The same is true for the weighted sum with methanol (Figure 6) in which all minima below the 175 meV peak are too pronounced. Also, contributions of the OH stretching band around 400 meV are missing in the spectrum of acetaldehyde after exposure. Similarly, using the spectrum of acetone for the construction of the weighted sum also fails to reproduce the experimental result as most obvious from the much too high intensity of the 63 meV band. A better agreement is found when a certain amount of the spectrum of a longer aldehyde, and more specifically of fresh propionaldehyde or exposed butyraldehyde, is added to that of fresh acetaldehyde (Figure 6). Here, the intensity fluctuates considerably less between the low-energy maxima and minima. Also, the 100 meV band has the correct shape. Although the
308 J. Phys. Chem. C, Vol. 111, No. 1, 2007
Figure 7. TDS curves recorded for 44, 28, 16, and 58 amu on multilayer films of acetaldehyde before (0 µC) and after electron exposure (2000 µC) at E0 ) 15 eV.
results from HREELS are not fully unambiguous, this result suggests that more complex products than postulated in reactions 1-3 are in fact formed with longer chain aldehydes being likely candidates. The next section will present results from TDS that shed more light on these reaction products. It must be noted that at longer exposure, due to secondary reactions, it becomes more difficult to identify a specific product. A general intensity increase at low loss energies, though, is reminiscent of recent results that hint toward the formation of longer chain compounds6,10 while unsaturated compounds can contribute to an intensity increase around 120 meV due to a characteristic CH wagging band6,11 and an apparent shift of the CdO stretching band to lower energy due to overlap with the CdC stretching band located slightly above 200 meV.11 3.2. TDS. Thermal desorption measurements were performed to identify product species produced in condensed acetaldehyde under exposure to low-energy electrons. E0 was chosen as 15 eV in all exposure experiments. It was verified upon dosing various gases into the vacuum chamber that mass spectra recorded with the present residual gas analyzer typically resemble those reported in ref 11. Therefore, in postirradiation TDS, we recorded the most intense unambiguous signals corresponding to various positive ion fragments characteristic of acetaldehyde as well as different anticipated reaction products as reported in ref 11. The upper two curves in Figure 7 depict the TDS curves of the parent ion of acetaldehyde (mass 44) before and after an electron exposure of 2000 µC. The maximum desorption rate of acetaldehyde is observed at ≈114 K with a small hightemperature shoulder at approximately 138 K attributed to the chemisorbed state. Comparison of desorption curves for undamaged and irradiated films recorded for 44 and 29 amu (HCO+, the major fragment in the acetaldehyde cracking pattern; data not shown) as well as for the minor fragments with masses 28 and 16 (included in Figure 7) indicates that the intensity of these
Swiderek et al. signals decays rapidly under irradiation. Electron exposure affects only the amplitude of the multilayer peak in the TDS spectrum, while preserving the peak position. Under exposure, new desorption peaks with maxima at 48 and 55 K appear in the TDS curves recorded for 28 and 16 amu. Through comparison with desorption of pure films, the desorbing species are identified as CO and CH4 that are expected from reactions 1 and 2. On the other hand, within the sensitivity of the present experiments there was no evidence for ethane desorption. Figure 7 also includes, on the bottom, TDS curves recorded for mass 58 amu. Here, a desorption signal centered around 140 K appears after exposure. The 58 amu value is also the mass of the parent ion of propionaldehyde. Alternative products with parent or fragment ions of the same mass include acetone. The assignment of the observed product thus requires further support. Therefore, various control experiments were performed to show that the desorption signal in the 58 amu curve at 140 K is due to desorption of propionaldehyde. First, a TDS curve was recorded for a nonexposed mixed multilayer reference film of acetaldehyde containing 4% propionaldehyde. While the signal is absent from nonexposed pure acetaldehyde, a desorption peak in the 58 amu curve around 140 K was observed for the reference film giving direct support for the described assignment. It must be noted that the total amount of propionaldehyde within the 4% mixture corresponds to the amount of vapor required for a submonolayer to monolayer film. This is deduced from the characteristic changes of the desorption temperature with increasing amount of propionaldehyde. While the monolayer desorbs at 140 K, thicker films or mixtures containing higher percentages of propionaldehyde lead to desorption signals between 125 and 130 K. Furthermore, from the intensity ratio between masses 44 at 114 K and 58 at 140 K (Figure 7) and the same ratio for the 4% reference film (not shown), we deduce that an exposure of 2000 µC produces less than 4% of propionaldehyde providing additional support for the conclusion that the 58 amu signal appearing at 140 K in exposed acetaldehyde is in fact due to this product. A more comprehensive quantitative analysis is in progress. To further discriminate propionaldehyde from other potential products, TDS curves for a number of different masses were recorded, including mass 43, which corresponds to the predominant (100% beside 63% of 58 amu) ion in the gas-phase cracking pattern of acetone but is absent in case of propionaldehyde,11 mass 31 (CH2O+, 1% in pure acetone, 6% in propionaldehyde, 5% in 2-propanol, 100% in 1-propanol11), and mass 45 (C2H4O+, 100% in 2-propanol11), and monitored during the TDS experiments following exposure to electrons. A 43 amu fragment in combination with a somewhat less intense 58 amu signal would thus hint toward formation of acetone. Unfortunately, the detection of such a signal is hampered by the intense desorption peak of remaining acetaldehyde as shown in the lower set of TDS curves of Figure 8. While an increased tail of this band points to the formation of new species after exposure, it is difficult to judge whether the 140 K desorption signal in the 58 amu curve is accompanied by a somewhat stronger signal in the 43 amu curve. A reference mixture of acetaldehyde containing 4% acetone, on the other hand, produces a desorption signal for mass 58 at 160 K that is again ascribed to desorption from monolayer or submonolayer coverage. Desorption from multilayer films or from mixtures containing 50% of acetone occurs at 130 K while a gradual shift of the desorption maximum is observed at intermediate coverages. These findings are reminiscent of earlier results for acetone adsorbed
Acetaldehyde under Exposure to Low-Energy Electrons
J. Phys. Chem. C, Vol. 111, No. 1, 2007 309 We must note that a desorption temperature of 168 K was previously reported for the multilayer phase of 1-propanol.24 This agrees reasonably with the present result. The same study, on the other hand, reported multilayer desorption of propionaldehyde of 152 K which is roughly 20 K higher than observed here. Multilayer desorption temperatures for acetone (132 K) and 2-propanol (155-158 K), on the other hand, again agree reasonably with the literature values of 133 K21 and approximately 155 K.22 The reasons for the strong discrepancy in the case of propionaldehyde are not clear at present. The temperature calibration may be more difficult at lower temperatures. Desorption temperatures of >32 and 37 K have recently been reported for the condensed phase and the second layer on Au of CO.25 More extensive control experiments shows that the desorption temperature of CO shifts toward a value of 38 K when the coverage is increased. This peak increases continuously with the leaked amount of vapor thus suggesting its assignment as multilayer signal. This suggests us to caution concerning the comparison of desorption temperatures from different sources. Ongoing work thus aims at extending our own database on desorption temperatures of various potential product species and establishing a more exact temperature scale.
Figure 8. TDS curves recorded for 31, 32, and 43 amu on multilayer films of acetaldehyde before (0 µC) and after electron exposure (2000 µC). The amplification factors refer to the 44 amu curves shown in Figure 7 at E0 ) 15 eV.
on C-precovered Pt(111).21 The small maximum observed at 158 K in the TDS curve for 43 amu after exposure is thus possibly due to formation of acetone. From a comparion with the intensities of desorption signals produced by the reference mixture and from the fact that the signal at 158 K in the TDS curve for 58 amu is considerably smaller than at 140 K, we estimate that the amount of acetone is considerably smaller than that of propionaldehyde. Again, the quantitative analysis will be reported in a forthcoming publication. A molecule with a 31 amu fragment desorbs at 156 K after exposure of condensed acetaldehyde (Figure 8, top). This fragment is characteristic of alcohols. As discussed below, formation of 2-propanol may be anticipated. On the other hand, the TDS curve for mass 45 (not shown) shows no distinct signal around 156 K while for 2-propanol a much stronger peak as compared to 31 amu is expected. Also, a mixture of acetaldehyde with 4% 2-propanol reveals that the monolayer desorption signal has a maximum at 173 K. A similar desorption temperature has also been reported previously for 2-propanol adsorbed on C-precovered Pt(111).22 The desorption temperature of the 31 amu fragment may indicate formation of methanol.23 A TDS experiment including 32 amu, which is expected with intensity similar to that for 31 amu of methanol,11 was thus performed but clearly shows that this signal is absent and formation of methanol can thus be ruled out (Figure 8, middle). Among other alcohols, ethanol should again show a dominant fragment ion with 31 amu but also a fragment 45 amu (50%).11 The lack of a 45 amu signal (not shown) at 156 K also argues against this assignment. On the other hand and as a last example, 1-propanol cannot be ruled out as 31 amu is the dominant fragment in this case.11 Control experiment using acetaldehyde mixed with 4% 1-propanol and pure 1-propanol, though, revealed a desorption temperature of 187 K for the monolayer of 1-propanol, while the multilayer film desorbs at 164 K. The origin of the 156 K desorption signal in the 31 amu curve thus remains unknown.
4. Discussion Depending on the incident energy, interactions with lowenergy electrons may lead to dissociation via electron attachment (DEA), electronic excitation, or dissociative ionization. DEA to acetaldehyde leading to production of CH3- has been reported to occur between E0 ) 6 and 7 eV,10 while O- was observed around 9.5 eV.26 Information on DEA at higher energies is not available. It can thus not be excluded but a discussion of the reaction pathways on this basis would be speculative. Neutral state reactions of acetaldehyde have been investigated using optical techniques and occur on the ground state and lowest excited state (1,3nπ*) potential energy surfaces, with highest reactivity in the triplet state, to yield, as major products, CH3 and CHO radicals.27 Finally, the ionization threshold is reported to be 10.2 eV in the gas phase and dissociative channels yielding C2H3O+ and H, CH3 and CHO+, CH4+ and CO, CH3+ and CHO, and CO+ and CH4, as well as CH3+, CO, and H, open up successively between 10.5 and 14.1 eV. While involvement of neutral dissociative reaction pathways cannot be excluded, due to the capability of low-energy electrons to excite triplet states,10 the increased reactivity at E0 above the ionization threshold suggests that dissociative ionization plays an important role. Assuming (1) that neutral excited-state and ionization reactions play the dominant role, (2) that a potentially formed radical CH3 is more reactive than a closed-shell positive ion CH3+, (3) ionization processes in the condensed phase are likely to take place at lower energy than in the gas phase due to polarization of the environment, (4) light and thus mobile species like H are most likely to induce intermolecular reactions, and (5) positive ions may also be neutralized again under continuous electron exposure, the following discussion reactions in acetaldehyde induced by electrons with E0 near 15 eV is based on CH3 and H radicals playing an important role in the formation of the observed products. The results described in the preceding section in fact give insight into the fate of the fragments remaining after production of CO under exposure to low-energy electrons according to a reaction with stochiometry as described by (1). Although the cooling capabilities in the HREEL experiment were probably insufficient to accumulate CH4, its formation as a major product is clearly demonstrated by the TDS results. This product very
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likely results from direct recombination of the radical fragments remaining after expulsion of CO from acetaldehyde. On the other hand, the formation of other products needs to be discussed in more detail as follows. The formation of ethane according to reaction 3 requires encounter of two methyl fragments. According to our own experience, the evaporation temperature of ethane is high enough to prevent it from desorbing from the molecular film. The missing evidence for ethane can be explained on statistical grounds. At the low current density applied in the present experiments, it is very probable that only isolated fragmentation occurs at any time. Quasi-simultaneous production of two adjacent CH3 fragments, as required for reaction 3, is thus likely to be rare. Adjacent H and CH3 fragments formed simultaneously upon release of CO, on the other hand, can recombine immediately or upon increase of the temperature during the TDS scan. In the scenario of isolated reactive electron scattering events, the fragments produced after release of CO are very likely to attack remaining acetaldehyde. Assuming H to be the more mobile fragment, it is expected that this fragment will be the major reactive species in the first step of the reaction. This may either lead to abstraction of an H from acetaldehyde which can take place in two positions •
H + CH3CHO f •CH2CHO + H2 •
H + CH3CHO f CH3C˙ O + H2
(4a) (5a)
or to attachment to the oxygen: •
H + CH3CHO f CH3C˙ HOH
(6a)
The different product radicals can, in the following step, recombine with the methyl fragment that was formed adjacent to the H radical. In the case of reaction 4a and assuming this fragment to be a radical, this produces propionaldehyde: •
CH2CHO + •CH3 f CH3CH2CHO
(4b)
This reaction explains the formation of longer chain aldehydes under electron exposure of condensed acetaldehyde as deduced from the analysis of the HREEL and TDS data. The analogous recombination reaction following (5a) would yield acetone:
CH3C˙ O + •CH3 f CH3COCH3
(5b)
On the other hand, the present experiments indicate that the formation of acetone is less favorable than formation of propionaldehyde. A possible explanation is that reaction 5a already is the initial step of subsequent abstraction of CO:
CH3C˙ O f •CH3 + CO
(5c)
It is thus possible that this step is faster than (5b) and thus represents a concurrent reaction to the formation of acetone. This reaction step is in fact a chain propagation reaction and would lead to formation of more than one CO per reactive electron scattering event. Also, in this scenario, formation of two methyl groups in close vicinity, as required for the formation of ethane, appears likely. The missing evidence of ethane formation thus suggests that either reaction 5c is not a dominant path or that these radicals predominantly react with hydrogen from the residual gas within the chamber to form methane which
in turn evaporates. On the other hand, if the latter was the case, formation of propionaldehyde should also be strongly suppressed. It must be noted that the CH bond at the aldehyde group is weaker than the CH bonds of the methyl group. This is deduced from the lower vibrational stretching frequency for the former19 but also from previous studies on gas-phase reactions of atomic hydrogen with acetaldehyde.28 On the other hand, in the condensed phase, the course of a reaction is subject to steric hindrance that can prevent a hydrogen radical from reaching the aldehyde group. From a statistical point of view, the probability for abstraction of hydrogen from the methyl group is three times higher than for the aldehyde group. Furthermore, reaction 4a does not lead to a possible competing CO releasing reaction as in the case of (5a). Therefore, recombination with the methyl group is in this case favored. This rationalizes the interpretation that propionaldehyde is the more important product compared to acetone. This leaves open the discussion of the third alternative, namely attachment of the hydrogen radical to the oxygen of an intact acetaldehyde according to reaction 6a. The subsequent recombination of the resulting radical with a methyl fragment would yield 2-propanol. We can speculate that this alcohol might dehydrate rapidly under continuing exposure yielding propene. Similar dehydration reactions under electron exposure have been observed for gaseous sugars.29 On the other hand, this scenario appears unlikely in the case of isolated electron-scattering events. Formation of another alcoholic species under prolonged exposure of acetaldehyde to the electron beam is not clear after the moderate exposures from the HREEL experiment but appears likely from the observation of mass 31 in TDS although the exact species could not be identified so far. Propene or similar products, on the other hand, might be formed as deduced from the broadening of the CdO stretching peak on the lowenergy side observed in HREEL spectra after prolonged exposure (compare Figure 3). Because of the missing alcohol OH stretching signal, which is expected above 400 meV as obvious from the spectrum of methanol (Figure 4), the formation of propene via this reaction pathway is rather unlikely. There is the possibility that unsaturated products are produced in subsequent H abstraction reactions once a sufficiently long carbon backbone has been formed.10 Nonetheless, this requires several reaction steps and is thus not expected at a significant rate during the initial stages of electron exposure of acetaldehyde. 5. Conclusions Reactions induced in condensed acetaldehyde by exposure to low-energy electrons have been investigated. HREEL spectra show that the acetaldehyde film is modified increasingly fast when the incident electron energy is increased above 10 eV. It is shown that CO is released at an energy of 14 eV. From a comparison with previous studies,4,5 we expect that this reaction is operative over a wider energy range around this value. The changes in the vibrational spectra have been analyzed in detail for an incident energy of 15.5 eV and hint toward formation of propionaldehyde. TDS experiments also give evidence for the formation of propionaldehyde beside the dominant products CO and CH4, which result from electron-induced expulsion of CO from acetaldehyde and subsequent recombination of the remaining fragments. Formation of acetone, on the other hand, is less favorable, and evidence for production of 2-propanol was not found.
Acetaldehyde under Exposure to Low-Energy Electrons The formation of propionaldehyde is rationalized by a reaction mechanism that is initiated by fragmentation of the acetaldehyde molecule yielding besides CO a CH3 and a H fragment. For statistical and steric reasons, attack of H on acetaldehyde is assumed to lead predominantly to abstraction of H from the methyl group. The resulting radical is then suggested to recombine with the remaining CH3 fragment to yield propionaldehyde. Abstraction of the aldehyde H atom and recombination can lead to formation of acetone, but the concurrent dissociation of the CC bond producing CO most probably slows down the rate of acetone formation. Acknowledgment. The authors thank S.-P. Breton and M. Michaud for technical support in recording the electronic spectra. P.S. thanks H. Metzner and his team at the mechanical workshop of the Institut fu¨r Physikalische Chemie at the University of Cologne for their support in building the TDS setup. References and Notes (1) Eck, W.; Stadler, V.; Geyer, W.; Zharnikov, M.; Go¨lzha¨user, A.; Grunze, M. AdV. Mater. 2000, 12, 805. (2) Antic, D.; Parenteau, L.; Sanche, L. J. Phys. Chem. B 2000, 104, 4711. (3) Cobut, V.; Frongillo, Y.; Patau, J. P.; Goulet, T.; Fraser, M.-J.; Jay-Gerin, J.-P. Radiat. Phys. Chem. 1998, 51, 229. (4) Lepage, M.; Michaud, M.; Sanche, L. J. Chem. Phys. 1997, 107, 3478. (5) Lepage, M.; Michaud, M.; and Sanche, L. J. Chem. Phys. 2000, 113, 3602. (6) Winterling, H.; Haberkern, H.; and Swiderek, P. Phys. Chem. Chem. Phys. 2001, 3, 4592. (7) Swiderek, P.; Deschamps, M. C.; Michaud, M.; Sanche, L. J. Phys. Chem. B 2004, 108, 11850-11856. (8) Antic, D.; Parenteau, L.; Lepage, M.; Sanche, L. J. Phys. Chem. B 1999 103, 6611.
J. Phys. Chem. C, Vol. 111, No. 1, 2007 311 (9) Ja¨ggle, C.; Swiderek, P.; Breton, S.-P.; Michaud, M.; Sanche, L. J. Phys. Chem. B 2006, 110, 12522. (10) Allan, M. J. Electron. Spectrosc. 1989, 48, 219. (11) NIST Chemistry WebBook, NIST Standard Reference Database Number 69; Linstrom, P. J., Mallard, W. G., Eds.; National Institute of Standards and Technology: Gaithersburg, MD 20899, Jun 2005 (http:// webbook.nist.gov). (12) Swiderek, P.; Deschamps, M. C.; Michaud, M.; Sanche, L. J. Phys. Chem. B 2003, 107, 563. (13) Ibach, H. Electron Energy Loss Spectrometers; Springer: Berlin, 1991. (14) Swiderek, P.; Winterling, H. Chem. Phys. 1998, 229, 295. (15) Wen, A. T.; Michaud, M.; Sanche, L. J. Electron. Spectrosc. 1998, 94, 23. (16) Lepage, M.; Michaud, M.; Sanche, L. J. Chem. Phys. 2000, 112, 6707. (17) Persson, B. N. J. Solid State Commun. 1977, 24, 573. (18) Ertl, G.; Ku¨ppers, J. Low Energy Electrons and Surface Chemistry; VCH: Weinheim, Germany, 1985. (19) Sanche, L.; Michaud, M. Phys. ReV. B 1984, 30, 6078. (20) Vibrations of acetaldehyde are reported in ref 11 as ν1 (CH3 d-str) 373 meV, ν11 (CH3 d-str) 368 meV, ν2 (CH3 s-str) 362 meV, ν3 (CH str) 350 meV, ν4 (CO str) 216 meV, ν5 (CH3 d-def) 179 meV, ν12 (CH3 d-def) 176 meV, ν6 (CH bend) 174 meV, ν7 (CH3 s-def) 168 meV, ν8 (CC str) 138 meV, ν9 (CH3 rock) 114 meV, ν13 (CH3 rock) 110 meV, ν14 (CH bend) 95 meV, ν10 (CCO def) 63 meV, and ν15 (torsion) 19 meV. (21) Dinger, A.; Lutterloh, C.; Biener, J.; Ku¨ppers, J. Surf. Sci. 1999, 437, 116. (22) Biener, J.; Lutterloh, C.; Schenk, A.; Po¨hlmann, K.; Ku¨ppers, J. Surf. Sci. 1996, 365, 255. (23) Ipolyi, I.; Swiderek, P. Unpublished results. (24) Brown, N. F.; Barteau, M. A. Langmuir 1992, 8, 862. (25) Gottfried, J. M.; Schmidt, K. J.; Schroeder, S. L. M.; Christmann, K. Surf. Sci. 2003, 536, 206. (26) Dressler, R.; Allan, M. J. Electron. Spectrosc. 1986, 41, 275. (27) Haas, Y. Photochem. Photobiol. Sci. 2004, 3, 6. (28) Ohmori, K.; Miyoshi, A.; Matsui, H.; Washida, N. J. Phys. Chem. 1990, 94, 3253. (29) Ptasinska, S.; Denifl, S.; Scheier, P.; Ma¨rk, T. D. J. Chem. Phys. 2004, 120, 8505.
