High-Resolution Vibrational Electron-Energy-Loss Spectroscopy of

J. Phys. Chem. C 2008, 112, 9405–9411

9405

High-Resolution Vibrational Electron-Energy-Loss Spectroscopy of Gaseous and Solid Glycine E. Burean,† R. Abouaf,‡,§ A. Lafosse,‡,§ R. Azria,‡,§ and P. Swiderek*,† UniVersita¨t Bremen, Fachbereich 2 (Chemie/Biologie), Leobener Straβe/NW 2, Postfach 330440, 28334 Bremen, Germany, and UniVersite´ Paris-Sud and Centre National de la Recherche Scientifique (CNRS), UMR 8625, Laboratoire des Collisions Atomiques et Mole´culaires, LCAM, Baˆtiment 351, UPS-11, Orsay, F-91405 France ReceiVed: January 16, 2008; ReVised Manuscript ReceiVed: March 10, 2008

The vibrational spectra of thin solid films of glycine deposited at temperatures between 12 and 130 K, as well as gaseous glycine, have been measured by high-resolution electron-energy-loss (HREEL) spectroscopy. The spectra are dominated by hydrogen-bonded glycine dimers when the films are deposited at low temperatures. The HREEL spectra of films deposited at higher temperature (130 K) have a different habitus that can be explained on the basis of a proton transfer reaction leading to the formation of zwitterionic glycine. Gas-phase HREEL spectra show bands characteristic of the neutral monomer of glycine. The results are discussed in relation to a previous study aiming at glycine synthesis in mixed ices under the effect of lowenergy electrons. Introduction The origin of the complex organic molecules that were necessary for the evolution of life on earth is a subject under debate. Although the formation of such species in the early earth atmosphere under the effect of electric discharges has been put forward as an explanation by the Miller experiments,1 the interstellar medium and, more specifically, mixed ices consisting of simple molecules on interstellar dust grains and exposed to radiation are also a potential source where synthesis may have taken place.2–4 In these chemical processes, low-energy secondary electrons abundantly produced under the effect of highenergy radiation may play an important role. Thus, it is interesting to study in detail the chemical reactions induced by low-energy electrons in mixed ices as well as their products. Experiments with low-energy electrons, on the other hand, pose specific problems. First of all, their penetration depth into the sample is limited, as the electrons can easily become thermalized by inelastic collisions and subsequently be trapped. Second, this often leads to charging of the sample. In consequence, studies of low-energy electron-induced reactions require ultrathin molecular films and methods capable of detecting very small amounts of material. Previously, high-resolution electron-energy-loss spectroscopy (HREELS) and thermal desorption spectrometry (TDS) have been put forward as suitable methods for the study of low-energy electron-induced reactions in condensed materials providing information on products and reactive cross sections.5–13 Both rely on the identification of species through characteristic signals that are optimally done by comparison with reference samples containing a certain percentage of an anticipated product.5,8,13 TDS is a fast method, but its interpretation is complicated by the fact that thermal reactions not directly related to the electron exposure contribute to the formation of products. Infrared spectroscopy in the reflection geometry (RAIRS), the latter being dictated by the need for a conductive substrate, suffers from a relatively low sensitivity as compared to HREELS. The potential †

Universita¨t Bremen. Universite´ Paris-Sud. § Centre National de la Recherche Scientifique. ‡

of HREELS for the detection of complex molecules thus deserves to be explored further. Glycine is the simplest aminoacid and, as such, has already been the subject of efforts to synthesize it from simple ice mixtures, both under the effect of ultraviolet radiation 4,14 and under high-energy electron irradiation.3 Its synthesis under the direct effect of low-energy electrons is, thus, of considerable interest. First efforts in this direction utilizing mixtures of ammonia and acetic acid were recently reported.10 On the other hand, although reference samples for product identification provide straightforward information in the case of simple molecules, aminoacids exist in various forms. These forms are, as additional complication, found in different amounts depending on the phase, temperature, and sample preparation method. This complicates the identification of glycine produced in mixed ices. Although gaseous glycine exists in the neutral form, the three known crystalline modifications (R, β, γ), each of which display a characteristic infrared spectrum,15 are composed of zwitterionic species (+H3NCH2COO-).16 Sublimation of glycine is thus accompanied by proton transfer between the carboxyl (COOH) group and the amino (NH2) group. In contrast to this, proton transfer is suppressed in the gas phase, and such neutral species can be trapped by matrix isolation, that is, codeposition of glycine with an inert gas whereby different conformers of the molecule can also be identified using infrared spectroscopy.17 This study revealed that glycine sublimated at 170 °C consists to 15% of species with an internal hydrogen bond, which shows a considerably reduced OH stretching frequency. Also, the possibility of trapping neutral glycine species in the solid state by deposition of gaseous samples onto a substrate cooled to very low temperature and the mechanism of proton transfer in the solid state after heating up the sample have been explored.16 It was shown that the formation of the zwitterionic form upon transition from the gas phase to the solid phase requires an activation energy. Although infrared spectra of the solid phase formed from glycine sublimated at 87 °C and condensed onto a surface cooled to 9 K resemble those obtained from R-glycine crystals, the spectrum changes considerably when the sublimation temperature is lowered to 64 °C.16 By comparison with

10.1021/jp800425n CCC: $40.75  2008 American Chemical Society Published on Web 05/29/2008

9406 J. Phys. Chem. C, Vol. 112, No. 25, 2008 DFT calculations of the spectrum, new infrared bands appearing under these conditions were assigned to a hydrogen-bonded carboxylic acid dimer.16 At very low temperatures, a neutral form of the molecule can thus also be trapped in a pure glycine solid. To contribute to the elaboration of a database for identification of glycine in HREELS experiments, we present here a comprehensive investigation of the HREEL spectra of glycine recorded under different conditions, both in the gas phase and in the condensed phase. This study most specifically puts effort into choosing conditions where catalytic surface effects that have been the subject of several previous studies 18–24 can be excluded. The present results are, thus, relevant for the conditions under which formation of molecules in interstellar ices may proceed. HREEL spectra were recorded for both gaseous glycine and glycine deposited on a surface cooled to temperatures between 12 and 130 K. Previous HREEL studies have mainly focused on the chemisorption of glycine on Si surfaces,24,25 and the spectrum of physisorbed glycine has been measured only at a surface temperature of 110 K.24 We thus also present the first study aiming at the HREEL spectral features of the different forms of glycine in the sub-monolayer regime. Experimental Section The experiments performed in Bremen were carried out in a µ-metal-shielded UHV chamber equipped with a HREEL spectrometer consisting of a rotating cylindrical double pass monochromator and a single pass electron-energy analyzer. Details of the experimental setup have been given elsewhere.26 The base pressure of the system reaches 10-11 mbar through the combined action of an ion pump and a titanium sublimation pump. The HREEL chamber is connected to a sample preparation chamber with a base pressure of about 2 × 10-8 mbar. Glycine (Serva, p.A.) deposition from the vapor phase onto a polycrystalline Pt substrate cooled by means of a closed-cycle He cryostat (Leybold) was performed using an evaporation source mounted in a load lock chamber with XYZ manipulator mated to the preparation chamber. The source thus travels into the preparation chamber through a gate valve. The load lock chamber is equipped with a separate ion pump capable of pumping down to a pressure of 10-7 mbar. The evaporation source consists of a crucible made from quartz glass with a resistive heater (Ta wire) and a chromel-alumel (type K) thermocouple for temperature measurement. A quartz crystal microbalance positioned in front of the crucible inside the load lock system is used to measure the sample deposition rate at room temperature. The microbalance is placed at the same distance as the Pt substrate used for samples deposition, a precise alignment being possible by means of the XYZ manipulator. Prior to each deposition, the Pt substrate was cleaned by resistive heating to an orange glow. After heating the crucible to a temperature around 80 °C, constant evaporation rates as measured with the microbalance could then be obtained. After reaching a constant evaporation rate the source was transferred into the preparation chamber. The glycine was deposited upon maintaining the Pt substrate at different temperatures ranging from 12 to 130 K. During a single deposition, the crucible was kept at the same position, and no adjustment of the heater current was made. The deposition rate measured at room temperature thus yields a lower limit for the actual deposition rate on the cold Pt substrate. Additional experiments were performed to study the influence of the substrate on the deposited glycine films by using an inert multilayer spacer of Xe (Messer Griesheim, 99.99%). The

Burean et al. desorption temperature of Xe films was reported between 62 and 67 K.27 Thus, at the lowest surface temperature used in the present measurements, desorption of the initially deposited Xe can be excluded. All HREEL experiments were carried out with the incident electron energy set to 5.5 eV after recooling the samples to 12 K and transferring the cryostat into the main chamber. The spectrometer was operated with a resolution of 9-12 meV, measured as the full width at half-maximum (fwhm) of the elastic peak for currents transmitted through the samples of the order of 0.1 nA. The typical collection time of a full spectrum was 30 min. Both the incidence and scattering angles were set at 60° from the surface normal. The experiments on gaseous glycine were performed in Orsay. Briefly, the device used for the present experiment is an electrostatic electron spectrometer using two hemispherical energy analyzers in tandem both in the gun and in the analyzer sections.28 The optics and magnetic shielding have been carefully designed to allow both the electron gun and the electron analyzer to go down to zero energy. In the present experiment, the incident electron beam energy was set to 1.9 eV and the electron energy loss spectrum was measured at a scattering angle of 90°. This takes advantage of the high vibrational excitation cross section associated to the resonance observed in electron transmission experiments by P. Burrow and co-workers.29,30 The electron current was ∼1 nA, and the achieved resolution was 29 meV, evaluated as the fwhm of the elastic peak. An effusive beam of glycine is produced by vaporizing commercial products (Merck) in a double stage tantalum oven, using a needle on top to allow a better definition of the collision center. After 48 h of outgassing at a temperature of 120 °C, the beam is obtained by heating to around 140 °C. The decomposition temperature of glycine was observed at 170-180 °C and was evidenced by recording the electronic excitation spectrum at 70 eV, where the characteristic bands of water were observed.31,32 To further check the neutral beam production, mass analysis of both positive and negative ions is achieved with a time-of-flight system based on a Mac-Laren geometry, the ions being collected onto microchannel plates. The wholeelectron spectrometer and time-of-flight system are heated to the same temperature to reduce insulating deposits. Results and Discussion Gas Phase HREEL Spectrum. The energy loss spectrum of gaseous glycine is presented in panel (A) of Figure 1, and the observed loss features are listed in Table 1. The achieved resolution is modest in comparison with optical spectroscopy, so that the observed features are broad and result from overlapping contributions of several modes. The assignments rely on a comparison with infrared spectra of the matrix-isolated neutral form of glycine. Furthermore, because there are discrepancies in literature concerning some of the assignments below 200 meV, we arbitrarily use those from ref 33. In the infrared spectra of matrix-isolated neutral glycine,17,33 the reported characteristic vibrations of the functional groups are a strong ν(OH) band at 441 meV (3560 cm-1), a weaker νas(NH2) band at 423 meV (3410 cm-1), a ν(CdO) band at 221 meV (1779 cm-1), and a δ(NH2) band at 202 meV (1630 cm-1). An additional signal at 397 meV (3200 cm-1) has been assigned as either νs(NH2)33 or ν(OH) of a glycine conformer with an internal hydrogen bond.17 Accordingly, the broad peak centered at 442 meV and the loss at 220 meV of the gas-phase electron energy loss spectrum are, respectively, attributed to the ν(OH) and ν(CdO) vibrational

Vibrational Electron-energy-loss Spectroscopy Glycine

Figure 1. (A) Vibrational electron-energy-loss spectrum of gaseous glycine (E0 ) 1.9 eV, ∆Efwhm ) 29 meV, θscattering ) 90°, energy step ) 5 meV). A minor nitrogen contribution was identified, and the energy loss observed at ∼293 meV is attributed to the excitation of the stretching mode of N2 molecules V ) 1. (B) Vibrational HREEL spectrum of 0.2 ML glycine on a multilayer spacer film of Xe, deposited and measured at 12 K (E0 ) 5.5 eV, ∆Efwhm ) 11 meV, θ0 ) θs ) 60°, energy step ) 1 meV).

modes of the -COOH group. The observed values confirm that most of the glycine molecules are in the neutral form, do not have internal hydrogen bond,17 and are not involved in hydrogen bonded dimers.16 The other contributions to the energy loss spectrum are attributed according to the IR data available in the literature for glycine in its neutral form. The contribution of the antisymmetric and symmetric stretching modes of the NH2 and CH2 groups leads, respectively, to the unresolved shoulder at about 420 meV and to the loss at 366 meV. The plateau extending from 167 up to 209 meV represents the contributions of the following overlapping modes: δ(NH2), δ(CH2), ω(CH2), tw(NH2), and δ(COH). The broad peak centered at 144 meV and ranging from 131 to 158 meV contains the contributions of the overlapping modes tw(CH2), ν(C-N), and ν(C-O). The loss observed at ∼67 meV is attributed to the overlap of the modes δ(COO), τ(C-O), ω(COO), and δ(CC)O). The modes ω(NH2), F(CH2), and ν(CC) contribute to the shoulder at about 95 meV. Solid Phase HREEL Spectra. Previous experiments on condensed glycine using HREEL spectroscopy have shown that charging of the sample and the resulting deflection of the electron beam can pose problems in recording vibrational spectra.34 In the case of condensed glycine molecules, it has been suggested that charging is caused by the interaction of slow electrons with the positive end (-NH3+) of the zwitterionic species present on the surface.34 The present HREEL experiments conducted at similar incident electron energy (5.5 eV) also reveal this effect, which becomes more severe when the glycine coverage is increased. The upper spectrum (A) in Figure 2, obtained for a monolayer (ML) of glycine deposited on the substrate held at 12 K, contains only very weak bands. In the case of thicker samples, the HREEL spectra showed only background noise above the elastic peak, pointing toward a

J. Phys. Chem. C, Vol. 112, No. 25, 2008 9407 severe charging that defocuses the electron beam to the point that only some elastic intensity remains. Equally, measurements at lower electron energies were not possible. Therefore, the thickness at which the measurements were performed had to be chosen and monitored carefully. Previous thermal desorption spectrometric (TDS) measurements on glycine deposited on graphite at 95 K34 showed desorption maxima shifting from approximately 295 K at submonolayer coverage to roughly 310 K at thickness approaching 2 ML. Similar to the present experiment, the thickness was monitored using a quartz crystal microbalance kept at room temperature. The sticking coefficient on the microbalance is thus lower than unity, and it is expected to be unity on the cryogenic substrate. Thus, we estimate that in both the previous TDS measurements34 as well as the present experiments the actual film thickness is somewhat larger than that estimated from the microbalance readout. The error is probably larger in the present experiments because deposition was performed at a rate that was approximately three times lower than in the previous study.34 At lower glycine coverage, charging is sufficiently reduced so that HREEL spectra can be acquired. The spectrum of 0.2 ML glycine deposited directly onto the substrate at 12 K is shown in Figure 2B, and the assignment of the bands is summarized in Table 1. To identify the actual form of glycine present in the adsorbate upon deposition, we first summarize in more detail the positions of the characteristic vibrations of the functional groups reported for the various forms. Infrared (IR) and Raman mode frequencies and assignments for different forms of glycine are listed in Table 1. There are still some discrepancies among all the available data due to mode intermixing, in particular in the attribution of the low energy modes. We made the choice to refer to the following data: matrix-isolated neutral glycine,17,33 zwitterionic form of glycine in the solid state,16 and glycine dimers as present in a lowtemperature solid film.16 The acid ionic form (+H3NCH2COOH) and anionic form (H2NCH2COO-) of glycine33 are not listed here since the associated characteristic modes cannot be differentiated from the zwitterionic and neutral forms on the basis of vibrational frequencies, considering the resolution that can be achieved in electron-energy-loss spectroscopy. The main features of IR spectra of matrix-isolated neutral glycine17 have been summarized in the previous section. The IR spectra of crystalline glycine reveal the presence of the zwitterionic species, as has been deduced from comparison with acid and alkaline salts of glycine.33 The characteristic group frequencies for the most common crystalline modification R-glycine have consequently been obtained as νas(NH3+) at 394 meV (3176 cm-1) and 377 meV (3040 cm-1), a broad and complex structure due to νs(NH3+) at and below 359 meV (2899 cm-1), a ν(COO-) band at 198 meV (1596 cm-1), and a δ(NH3+) band at 200 meV (1615 cm-1). In addition, Raman spectra revealed a higherfrequency δ(NH3+) band at 208 meV (1676 cm-1).33 Finally, IR spectra of glycine films deposited at 9 K suggest the presence of neutral carboxylic acid-like glycine dimers when the sublimation temperature is sufficiently low (64 °C).16 Characteristic bands in this case have been reported as ν(OH) at 417 meV (3365 cm-1), νas(NH2) at 410 meV (3310 cm-1), νs(NH2) at 399 meV (3220 cm-1), ν(CdO) at 213 meV (1715 cm-1), and δ(NH2) at 204 meV (1643 cm-1). Zwitterionic glycine very similar to R-glycine was obtained by increasing the sublimation temperature to 87 °C. In the range of characteristic group frequencies, the present spectra of sub-monolayer glycine deposited at 12 K (Figure 2B)

9408 J. Phys. Chem. C, Vol. 112, No. 25, 2008

Burean et al.

TABLE 1: Assignment and Energy (Given in meV) of the Bands Measured for Neutral Glycine in the Gas Phase, Neutral Dimers, and Zwitterionic Glycine Deposited in Submonolayer Quantity at 12 and 130 Ka this work

literature crystalline (zwitterionic)

gas

solid 12 K

solid 130 K

442 420 (urs)

matrixb (neutral) IR17,33

IR33

Raman33

low-T solid film (carboxylic acid dimers) IR16

441 423 418

417 410 399 397

assignments16,33 ν(OH) νa(NH2) ν(OH) νas(NH2) νs(NH2) νs(NH2) 33 or

ν(OH) in H-bonded conformer 17

366

367

370 365

220

367 368 221

394, 377 359c 373 369

νs(CH2)

214 209-167 (urs) 198 177

213 202 202, 201

178

177

200 198 189, 187 181, 179 175, 174

195 189, 187 181, 179 175, 174

165 162

165 165

208, 203 204

172 170 166 167 158-131

157 140

142 140

143 140, 138

142, 138

128

129

137 130 130

125 115

νas(NH3+) νs(NH3+) νas(CH2)

390 361, 358 374

114 111

113 111 109

δas(NH3+) δ(NH2) νas(COO-) δs(NH3+) δ(CH2) νs(COO-) ω(CH2) tw(NH2) δ(COH) ω(CH2) tw(CH2) 155 tw(CH2) ν(C-N) F(NH3+) ν(C-O) ν(C-N) ν(C-O)

ν(C-O) + δ(COH)

116

ν(CC) + ω(NH2) F(CH2)

115 ν(CC) 107

107, 106 99

95(urs) 84-53 (max. at 67)

82 80

79

87

75 64

75 63 60 62

87

77 64

64 62

62 32

44 23, 21

83 δ(COO-) τ(C-O) ω(COO-) τ(CN)

ω(NH2) ω(NH2) + ν(CC) F(CH2) ν(CC) δ(COO)

ω(COO) δ(CCO) δ(CCdO)

57 30

ν(CdO) (CdO)

45 τ(CC)

δ(NCC)

a

IR and Raman data reported in the literature for different forms of glycine are reported for comparison: matrix-isolated neutral glycine,17,33 zwitterionic form of glycine in the solid phase,33 glycine dimers as obtained in low-temperature solid films.16 Key: a, asymmetric; s, symmetric; ν, stretching; δ, bending; ω, wagging; F, rocking; tw, twisting; τ, torsion; max, maximum; urs, unresolved shoulder. b Reference 17 shows a spectrum with more fine structure; assignments are partially conflicting with reference 33; several bands in the fingerprint region are also suggested to be due to vibrations of minor conformers. c Highest member of complex structure.

show a band with maximum at 418 meV and a shoulder on the low-energy side, the ν(CH2) band at 367 meV, broad signals between the latter two bands and below ν(CH2), as well as a band with maximum at 199 meV and a shoulder at approximately 214 meV (for other bands, see Table 1). These

values show a close agreement with the data for the neutral carboxylic acid-like glycine dimers.16 Moreover, the present band positions match reasonably well with those obtained by IR spectroscopy from multilayers of glycine deposited on Cu (110) at 85K by Barlow et al.18

Vibrational Electron-energy-loss Spectroscopy Glycine

Figure 2. Vibrational high-resolution electron-energy-loss spectrum of (A) 1 ML glycine, (B) 0.2 ML glycine, and (C) 0.2 ML glycine on Xe spacer layers, deposited and measured at 12 K.

Although some contribution of zwitterionic glycine can not be excluded, the assignment of the observed bands to neutral glycine dimers is reasonable because the deposition conditions are similar to the previous work.16 The sublimation temperature measured in the present experiments was near 80 °C, but the actual temperature of the glycine powder was probably somewhat lower because the heating wires were nearer to the thermocouple than to the sample. To exclude an influence of the substrate on the adsorbed glycine, control experiments were performed in which glycine was deposited at 12 K on top of a multilayer spacer film of Xe (Figure 2C). The spectrum agrees with the one obtained upon direct deposition on the substrate, thus demonstrating that the presently used Pt is chemically inert. This is due to the substrate cleaning procedure by simple resistive heating of the metal, which produces carbon residues by thermal decomposition of the molecular films.26 Also, the probability of charge transfer between the substrate and adsorbed glycine molecules is expected to be modified by the presence of a spacer layer. The close similarity of the spectra of glycine deposited on the spacer layer and directly on the substrate shows that this effect is negligible. In addition, in Figure 1, the spectrum attributed to glycine dimers deposited on a Xe spacer (Figure 2C) is directly compared to the gas phase spectrum ascribed to vibrational excitation of neutral glycine. The band attributed to ν(OH) shifts from 442 meV in the gas phase to 418 meV, and the loss attributed to ν(CdO) shifts from 220 to 214 meV, as expected upon hydrogen bonding to form the dimers. A detailed comparison of the low-energy part of the energy-loss spectra is complicated by the difference in resolution and electron energy at which the spectra were recorded. Upon low-energy electron scattering, glycine behaves very similarly to alanine in the gas phase.32 In analogy to results for alanine obtained at 6 eV, the intensity of ν(CdO) is expected to decrease and that of ν(CH2) to increase upon increase of the incident electron energy to 1.9

J. Phys. Chem. C, Vol. 112, No. 25, 2008 9409

Figure 3. Vibrational high-resolution electron-energy-loss spectra of sub-monolayer glycine films deposited at different temperatures.

eV, where scattering is resonant32 to 5 eV. The higher intensity of ν(CdO) in the gas phase spectrum (Figure 1A) as compared to the solid phase result (Figure 1B) is in accord with this effect. The HREEL spectrum of sub-monolayer glycine changes upon increase of the substrate temperature (Figure 3A). At 130 K, it closely resembles results obtained for physisorbed glycine deposited at 110 K on Si(111) and assigned to the zwitterionic form.24 Prominent bands at 370 and 202 meV are thus ascribed to ν(CH2) and δ(NH3+). Further signals above 200 meV include a broad band around 235 meV that can be assigned to a multiple scattering process involving the excitation of the bands at 202 and 33 meV, to a weak and structureless intensity increase below the ν(CH2) band that covers the range of the νs(NH3+) band in zwitterionic glycine, and to a broad and equally structureless feature above the ν(CH2) band. This may be due to νas(NH3+) as well as some remaining neutral glycine. Most importantly, ν(CdO) can no longer be located, suggesting that zwitterionic species are predominant now. Finally, the close correspondence of the present solid phase HREELS spectra and previous data on solid glycine suggest that in the present samples island formation plays an important role. This also rationalizes the pronounced charging effects. Comparison of the Spectra and Implications for Glycine Identification in the Mixed Ices. The chemical reactivity induced by low-energy electron irradiation was recently investigated by HREELS in a mixture of partially deuterated acetic acid and ammonia CH3COOD:NH3 (1:1) condensed as multilayers at 25 K.10 In addition to consumption of acetic acid and ammonia, characteristic and strong new bands in the HREEL spectrum at 62 and 140 meV were observed after irradiation at 20 eV using a dose of 7 × 1016 electrons/mm2. It was suggested that these new bands relate to formation of glycine. Because of the lack of literature HREELS data on intact glycine molecule at that time in the literature, it was only possible to compare the pattern of the observed new losses to Raman and IR data. Because of the very small intensity signal at 214 meV (ν(CdO)), a zwitterionic form of glycine was assumed. The 62 meV band was shown to coincide with the δ(CCO) and τ(CN) mode of

9410 J. Phys. Chem. C, Vol. 112, No. 25, 2008 zwitterionic glycine, whereas the 140 meV band would agree with the F(NH3+) band. Because of the strong band overlap with the remaining NH3, contributions of other glycine forms (neutral and anionic) could not be excluded. It must be mentioned that a shift to lower energy losses of the ν(CH) band was also observed. This shift is consistent with a loss of CH3 groups and with larger contributions by CH2 groups as expected for glycine formation. Intensities in HREEL spectra are often very different from those in IR or Raman spectra, due to contributions of different inelastic electron scattering mechanisms (dipolar, resonant, or impact scattering). Therefore, this study also aims at performing a closer comparison with the new bands observed in the irradiated mixture of acetic acid and ammonia. On the other hand, the present results show that the observed spectra strongly depend on the preparation conditions of the sample (phase, film deposition conditions) and thus on the molecular environment and orientation. Nonetheless, and in accord with the previous interpretation, the band at 62 meV is wellreproduced by the present HREEL spectrum of glycine deposited at 130 K (Figure 3), which is ascribed to a phase dominated by zwitterionic species. In contrast, the equally strong bands at 32 and 202 meV observed in the same spectrum were not found in the case of the reaction mixture. Equally, the spectra of both glycine deposited at 130 K (dominantly zwitterionic) and at 12 K (more neutral species) do not show a strong band at 140 meV. This discrepancy, on the other hand, may be explained as follows. The reaction mixture,10 that is, the sample after electron exposure, still shows important contributions by intact ammonia molecules (in particular, intact bands at about 135, 204, and 421 meV) while much less of the remaining acetic acid is visible. Any product formed in the mixture is, thus, most likely embedded in an environment with considerable basicity. Thus, glycine is expected to be in an anionic (H2NCH2COO-), rather than a zwitterionic, form. As seen in Table 1, both the band at 32 meV and the one at 202 meV are related to or contain strong contributions from the amino group of glycine. If the molecule is present in the anionic form, amino-related vibrations, and thus their excitation bands, should be more like those of neutral glycine and thus compare more closely to those found in the spectrum of glycine deposited at the lower temperatures (Figure 2). Here, the intensity at these energy losses is much less pronounced, similar to that observed in the reaction mixture. The presence of anionic glycine can, thus, present an explanation for the lack of correspondence between the spectrum of the reaction mixture and those of solid glycine. Finally, although contributions from other products can not be ruled out, we also cannot exclude that the anionic form of glycine would give rise to a much more intense band at 140 meV than the neutral (dimer) or zwitterionic form. To come to a more substantial conclusion, the spectra obtained by low-energy electron irradiation of ices must be compared to the HREEL spectra of glycine embedded in such complex matrices. Even in such a case, varying the initial proportions of the acid/base mixture could favor the formation of glycine either in its acidic (+H3NCH2COOH) or in its anionic form (H2NCH2COO-). This must be further explored. Conclusion In condensed phase experiments, vibrational HREEL spectra for the dimer form, consisting of two neutral molecules linked by hydrogen bonds involving their car-

Burean et al. boxylic acid function, and for the zwitterionic form of glycine have been obtained. The intensity distribution and the band positions are characteristically different in the two cases. Nonetheless, none of them displays a strong band at 140 meV that could explain the new band observed in mixed NH3/ CH3COOD ices after exposure to low-energy electrons if they are assumed to be due to zwitterionic glycine. Anionic glycine, on the other hand, appears to be a more likely product on the basis of the present findings. This is consistent with the fact that any glycine formed in this mixture is still embedded in NH3 and possibly remaining CH3COOD. The ultimate conclusion about the formation of glycine in these ices thus requires HREEL spectra obtained for glycine embedded in such a matrix. This must be further explored. Acknowledgment. The authors acknowledge the financial support provided by the Cost Action CM0601 Electron Controlled Chemical Lithography (ECCL) and the ESF program Electron Induced Processes At the Molecular Level (EIPAM). References and Notes (1) (a) Miller, S. L. Science 1953, 117, 528. (b) 1959, 130, 245. (2) Greenberg, J. M. Surf. Sci. 2002, 500, 793. (3) Holtom, P. D.; Bennett, C. J.; Osamura, Y.; Mason, N. J.; Kaiser, R. I. Astrophys. J. 2005, 626, 940. (4) Mun˜oz Caro, G. M.; Meierhenrich, U. J.; Schutte, W. A.; Barbier, B.; Arcones Segovia, A.; Rosenbauer, H.; Thiemann, W.H.-P.; Brack, A.; Greenberg, J. M. Nature 2002, 416, 403. (5) Lepage, M.; Michaud, M.; Sanche, L. J. Chem. Phys. 2000, 113, 3602. (6) Go¨o¨tz, B.; Winterling, H.; Swiderek, P. J. Electron Spectrosc. 1999, 105, 1. (7) Swiderek, P.; Ja¨ggle, C.; Bankmann, D.; Burean, E. J.Phys.Chem. C 2007, 111, 303. (8) Burean, E. Swiderek, P. Surf. Sci. in press. (9) Ipolyi, I.; Michaelis, W.; Swiderek, P. Phys. Chem. Chem. Phys. 2007, 9, 180. (10) Lafosse, A.; Bertin, M.; Domaracka, A.; Pliszka, D.; Illenberger, E.; Azria, R. Phys. Chem. Chem. Phys. 2006, 8, 5564. (11) Sedlacko, T.; Balog, R.; Lafosse, A.; Stano, M.; Matejcik, S.; Azria, R.; Illenberger, E. Phys. Chem. Chem. Phys. 2005, 7, 1277. (12) Orzol, M.; Sedlacko, T.; Balog, R.; Langer, J.; Karwasz, G. P.; Illenberger, E.; Lafosse, A.; Bertin, M.; Domaracka, A.; Azria, R. Int. J. Mass Spectrom. 2006, 254, 63. (13) Ja¨ggle, C.; Swiderek, P.; Breton, S.-P.; Michaud, M.; Sanche, L. J. Phys. Chem. B 2006, 110, 12512. (14) Nuevo, M.; Meierhenrich, U. J.; Mun˜os Caro, G. M.; Dartois, E.; d’Handecourt, L.; Deboffle, D.; Auger, G.; Blanot, D.; Bredeho¨ft, L.; Nahon, L. Astron. Astrophys. 2006, 457, 741. (15) Chernobai, G. B.; Chesalov, Yu.A.; Burgina, E. B.; Drebushchak, T. N.; Boldyreva, E. V. J. Struct. Chem. 2007, 48, 332. (16) Gomez-Zavaglia, A.; Fausto, R. Phys. Chem. Chem. Phys. 2003, 5, 3154. (17) Stepanian, S. G.; Reva, I. D.; Radchenko, E. D.; Rosado, M. T. S.; Duarte, M.L.T.S.; Fausto, R.; Adamowicz, L. J. Phys. Chem. A 1998, 102, 1041. (18) Barlow, S. M.; Kitching, K. J.; Haq, S.; Richardson, N. V. Surf. Sci. 1998, 401, 322. (19) Efstathiou, V.; Woodruff, D. P. Surf. Sci. 2003, 531, 304. (20) Lo¨fgren, P.; Krozer, A.; Lausmaa, J.; Kasemo, B. Surf. Sci. 1997, 370, 277. (21) Qu, Y.-Q.; Wang, Y.; Li, J.; Han, K.-L. Surf. Sci. 2004, 569, 12. (22) Tzvetkov, G.; Ramsey, M. G.; Netzer, F. P. Surf. Sci. 2003, 526, 383. (23) Tzvetkov, G.; Koller, G.; Zubavichus, Y.; Fuchs, O.; Casu, M. B.; Heske, C.; Umbach, E.; Grunze, M.; Ramsey, M. G.; Netzer, F. P. Langmuir 2004, 20, 10551. (24) Huang, J. Y.; Ning, Y. S.; Yong, K. S.; Cai, Y. H.; Tang, H. H.; Shao, Y. X.; Alshahateet, S. F.; Sun, Y. M.; Xu, G. Q. Langmuir 2007, 23, 6218. (25) Lopez, A.; Heller, T.; Bitzer, T.; Richardson, N. V. Chem. Phys. 2002, 277, 1. (26) Swiderek, P.; Winterling, H. Chem. Phys. 1998, 229, 295. (27) Widdra, W.; Trischberger, P.; Frieβ, W.; Menzel, D. Phys. ReV. B 1998, 57, 4111.

Vibrational Electron-energy-loss Spectroscopy Glycine (28) Abouaf, R.; Pommier, J.; Dunet, H.; Quan, P.; Nam, P.-C.; Nguyen, M. T. J. Chem. Phys. 2004, 121, 11668, and references therein. (29) Scheer, A. M.; Mozejko, P.; Gallup, G. A.; Burrow, P. D. J. Chem. Phys. 2007, 126, 174301. (30) Aflatooni, K.; Hitt, B.; Gallup, G. A.; Burrow, P. D. J. Chem. Phys. 2001, 115, 6489. (31) Lassettre, E. N.; Skerbele, A.; Dillon, M. A.; Ross, K. J. J. Chem. Phys. 1968, 48, 5066.

J. Phys. Chem. C, Vol. 112, No. 25, 2008 9411 (32) Abouaf, R. Chem. Phys. Lett. 2008, 451, 25. (33) Rosado, M. T.; Duarte, M.L.T.S.; Fausto, R. Vibr. Spectrosc. 1998, 16, 35. (34) Lo¨fgren, P.; Krozer, A.; Chakrov, D. R.; Kasemo, B. J. Vac. Sci. Technol. A 1998, 16, 2961.

JP800425N