Metastable Impact Electron Spectroscopy (MIES) and Reflection

Jul 10, 2007 - The interaction of cyclic dimers of acetic acid with amorphous solid water (D2O) on polycrystalline silver was investigated. Metastable...
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J. Phys. Chem. C 2007, 111, 11302-11313

Metastable Impact Electron Spectroscopy (MIES) and Reflection Absorption Infrared Spectroscopy (RAIRS) Study of Acetic Acid Interacting with Amorphous Solid Water S. Bahr and V. Kempter* Technische UniVersita¨t Clausthal, Institut fu¨r Physik und Physikalische Technologien, Leibnizstr. 4, D-38678 Clausthal-Zellerfeld, Germany ReceiVed: January 16, 2007; In Final Form: March 15, 2007

The interaction of cyclic dimers of acetic acid with amorphous solid water (D2O) on polycrystalline silver was investigated. Metastable impact electron spectroscopy (MIES) was utilized to obtain information on the electronic structure of the outermost surface. Reflection absorption infrared spectroscopy (RAIRS) allowed for the identification of the acetic acid species and provided information on their hydration and the waterinduced fragmentation. The interpretation of the results required also the study of the interaction of acetic acid with the neat silver substrate. On amorphous solid water (ASW), the acetic acid species quickly saturate the dangling OD groups present at the water surface at a coverage of less than 0.5 layers. In the first layer, the cyclic acid dimers break up under the influence of their interaction with the D2O molecules. For coverages >0.5 layers, strong lateral interaction between the acid molecules is responsible for the completion of the first adlayer. The second and following layers formed at 124 K consist of cyclic dimers. The splitting of the cyclic dimers by their interaction with the amorphous solid water, followed by their hydration, allows for a detailed characterization of the various acetic acid species resulting from the acetic acid-water interaction.

1. Introduction The structure of acetic acid (in the following denoted by AA) in the gas (cyclic dimers) or the solid phase (chains) is well understood.1-3 However, this is not the case for its structure in aqueous solutions or in interaction with ice surfaces, although the local structures of AA in water have received considerable attention recently.4-7 The study of the uptake of oxygenated hydrocarbons, including AA, is important for atmospheric chemistry because of their decomposition into hydroxyl radicals, which are thought to be responsible for ozone depletion. Ice as a surface for AA uptake has received attention because of the relevance of its surface chemistry to processes found in environmental science.8-10 Several theoretical works have appeared in the past years dealing with the AA-water (including ice) interaction: The H-bonding interactions in AA mono- and dihydrates have been studied with density functional theory (DFT).6 Similar work is available for formic acid mono- and dihydrates.11 It is of interest for the present work that the binding energies per H-bond of the most stable conformers are larger than those of the water dimer.6 A DFT study of the vibrational properties of acetic acid monomers and dimers was reported in ref 12. The most stable structures in the neat acid and the microhydrated environment were studied in a combined ab initio chemical and molecular dynamics (MD) simulation.13 It was concluded that the cyclic dimer structure does not appear in aqueous solutions; instead, water causes the break-up of the C-O-H‚‚‚O bonds leading to water-separated ringtype structures, containing both AA and water molecules. For carboxylic acids monomers (formic and acetic acid) interacting with ice surfaces, free energy profiles obtained from * To whom correspondence should be addressed. E-mail: [email protected]. Tel: +49-5323-72-2363. Fax: +49-532372-3600.

classical MD calculations demonstrate that these acids are strongly trapped at the ice surface at 250 K.14 Caused by the ability to form strong H-bonds with the ice surface and among themselves, the adsorption process can be quite complicated: on the one hand, it is predicted that the interaction of isolated AA species (mostly cyclic dimers in the gas phase) with ice leads to the dissociation of AA dimers,9 but on the other hand the interaction between adsorbed monomer species near monolayer coverage of the ice surface may lead to the reformation of AA oligomers.9 DFT computations for various AA clusters, monomers to trimers both unhydrated and hydrated in interaction with ice, have suggested that the most likely structure resulting from the interaction of AA on ice films are cyclic dimers.15 Experimentally, the identification and characterization of local structures in the neat liquid and aqueous AA solutions is hampered by the fact that the structures from the various vibrational modes, the carbonyl stretch νCdO in particular, are not very-well defined on the one hand, and structures from different oligomers, cyclic and linear dimers as an example, overlap.5,4 Thus, no consensus exists as for the dominating species, cyclic dimers or chainlike structures, in the neat liquid and aqueous AA solutions. It was demonstrated recently that comparatively sharp vibrational bands appear during the adsorption of AA on amorphous solid water (ASW) and polycrystalline ice.10 While demonstrating that the AA-water interaction is due to H-bond formation between the AA carbonyl group and the OH dangling at the ice surface, no attempt was made to actually identify the AA species in contact with the ice film. Other techniques, such as metastable impact electron spectroscopy/ ultraviolet photoelectron spectroscopy (MIES/UPS7) and timeof-flight-secondary-ion mass spectrometry (TOF-SIMS16), provided valuable information on the surface activity of the AA species and hydration phenomena, but again failed to give detailed information on the identity of the existing AA-water complexes. Because previous work has indicated that the

10.1021/jp0703556 CCC: $37.00 © 2007 American Chemical Society Published on Web 07/10/2007

MIES and RAIRS Study of Acetic Acid with D2O

Figure 1. UHV chamber with VUV and X-ray source (a), MIES source (b), electron gun (c), CHA (d), Quadrupole mass spectrometer (e), RAIRS spectrometer (f), KBr windows (g), MCT detector (h), sputter ion gun (i), and sample transfer system (j).

combination of the electron spectroscopy MIES with reflection absorption infrared spectroscopy (RAIRS) (with submonolayer sensitivity) may be a promising approach, we have in the present work employed this combination to obtain, together with the results from theoretical modeling,9,15 detailed information on the lateral interaction among the AA species themselves and with water. RAIRS is applied to identify the adsorbed or embedded AA-water complexes, while MIES provides information on the electronic structure of the films as well as on the location of the AA-water species within the film. 2. Experimental Methods The ultrahigh vacuum (UHV) apparatus has been described in connection with our previous studies on the interaction of carboxylic acids with ASW.7,17 Some details and the important specifications of all employed facilities, in particular of the newly added RAIRS setup, are given in the following. 2.1. Electron Spectroscopy. The present setup is illustrated in Figure 1. It consists of a stainless steel UHV chamber made by Varian Inc. with a base pressure of 5E-10 Torr; the upper level of the chamber is shown in the Figure 1. The lower level contains a turbomolecular and a titanium sublimation pump, together with the pressure gauges and dosing valves of the gas inlet system. The sources providing photons for photoelectron spectroscopy measurements are: an Omicron HIS-13 vacuum ultraviolet (VUV) source, providing HeI (21.2 eV) and HeII (40.8 eV) photons, an X-ray source made by Fisons, equipped with a dual anode providing non-monochromated Al-KR and Mg-KR photons (Figure 1a). A self-made cold-cathode discharge

J. Phys. Chem. C, Vol. 111, No. 30, 2007 11303 source produces metastable He atoms with 19.8 eV potential energy (the potential energy of He* (23S)) for MIES and HeI photons (21.2 eV) for UPS (Figure 1b). Through the use of a TOF technique to separate the HeI photons from the metastable atoms, it is possible to record MIES and UPS spectra at the same time. The metastable He atoms approach the surface with near-thermal kinetic energy (60-100 meV) and interact with the outermost surface layer by Auger processes. This makes MIES an extremely surface sensitive and nondestructive technique (for a detailed introduction into MIES, see refs 18 and 19 and references given therein). The chamber is furthermore equipped with an electron gun (VSW EG-5) to check the sample surface for contamination by auger electron spectrsocopy (AES) (Figure 1c). The abovementioned probes (electrons, photons, and metastables) interact with the surface under investigation by releasing electrons from different escape depth of the upper surface region. The emitted electrons are analyzed with a Leybold EA10/100 concentric hemispherical energy analyzer (CHA) (Figure 1d). In the present study, UPS was used mainly to control the film growth. 2.2. Infrared Spectroscopy (RAIRS). The grazing incidence reflection RAIRS setup contains a Bruker IFS 66v/S vacuum Fourier transform infrared spectrometer, connected directly to the UHV chamber (Figure 1f). The mid-IR beam, produced by a SiC glowbar, is focused by a KBr lens and enters the chamber trough a differentially pumped KBr window (Figure 1g). It is reflected on the polycrystalline silver disc under an angle of 83.5° to the surface normal and leaves the chamber through another KBr window. After passing a second KBr lens, it is focused by a parabolic mirror onto a liquid nitrogen-cooled mercury cadmium telluride (MCT) detector (Infrared Associates Inc.), housed in a pumped stainless steel chamber (Figure 1h). The detector cutoff is at 700 cm-1; thus, the region from 4000 cm-1 down to 700 cm-1 can be used for interpretation of the vibrational spectrum of the adsorbate. Normally, the spectra were recorded with a resolution of 4 cm-1 and 1000 and/or 270 scans were added prior to the Fourier transformation and calculation of the absorbance as the negative logarithm of the ratio of a spectrum with the adsorbate to a spectrum of the clean silver disc. The spectrometer, the detector chamber, and the whole beam line can be pumped down to 3 Torr to minimize absorption of the infrared photons in the water and CO2 region. 2.3. Preparation of the Samples. The substrate used in this investigation is a polished polycrystalline Ag disc with a diameter of 20 mm, made from a rod supplied by Goodfellow Inc. with a purity of 99.95%. This disc is positioned in an exchangeable sample holder, which is mounted to an XYZ manipulator. The sample can be heated via a W filament behind the Ag disc or by electron bombardment. It can be cooled down to 124 K by liquid nitrogen-cooling facilities connected to the manipulator. The temperature was monitored with a W-Re (26-5%) thermocouple, clamped to the backside of the sample. Prior to the measurements, the Ag disc was sputtered with 2000 eV Ar+ ions at 500 K for periods of 30 min, followed by briefly annealing the sample to 750 K. The sample cleanliness was checked with AES after each cleaning cycle. Deuterium oxide (D2O 99.8%) was supplied by Alfa Aeser, glacial acetic acid (CH3COOH 99.99+%) and deuterated acetic acid, denoted by AA-d1 (CH3COOD 99%), by Sigma Aldrich. All chemicals were purified by several freeze-pump-thaw cycles prior each use. The substrate, held at 124 K, was exposed to the vapor of the liquids by backfilling the chamber through different leak valves to prevent cross contamination in the gas line. For the

11304 J. Phys. Chem. C, Vol. 111, No. 30, 2007 adsorption of D2O, a pressure of 4E-8 Torr and 1.5E-8 Torr for AA and AA-d1 respectively, was used. All exposures are stated in Langmuir (L) (1 L ) 1 E-6 Torr s-1). The cleanliness of the employed liquids was checked by monitoring their mass spectra during adsorption measurements. The use of D2O allows us to get information on the AAwater interaction from the study of the H/D exchange.16 Moreover, it facilitates the comparison of the present results with those from the literature.4,16,20 When D2O hits the chamber walls prior to adsorption on the sample, an H/D exchange with surface OH groups at the chamber walls can take place producing contaminations of the adsorbing D2O by H2O. This results in a small feature in the RAIRS spectra of D2O at 3300 cm-1. This problem applies also for AA-d1. The exposure of D2O was calibrated against the stretching mode of the free OD group of D2O at the solid water surface (dangling OD, denoted by dOD), located at 2728 cm-1, while adsorbing D2O on the clean silver surface. The saturation of the dOD band appears at the same exposure as the saturation of the work function decrease, measured with MIES and UPS. On this basis, we conclude that the first water-bilayer has formed after an exposure of 9 L of D2O. According to RAIRS, the film is amorphous and thereafter denoted by ASW.20 Furthermore, under the chosen conditions the ASW film possesses a very low porosity.16,21 With an stagnation pressure of 11.5 Torr in the AA gas inlet system held at room temperature and the known equilibrium constant, Keq for 298.15 K,22 for the dimerization reaction at this temperature one can estimate the AA dimer fraction xdim in the gas phase by the following relation9

Keq )

xdim p0 2p x T mono

Here, xdim and xmono are the molar fractions of monomers and dimers, respectively, and p0/pT is the total pressure of the mixture in units of the normal pressure. Under our experimental conditions, the gas phase contains 82% dimers, neglecting the possibility that the molar fractions could become modified by wall collisions of the AA species, inside the vacuum chamber, prior to adsorbtion on the cold substrate. For the codeposition measurements, D2O and AA were deposited by backfilling at 124 K at a pressure ratio of 1:1 (as determined mass spectroscopically). 3. Results 3.1. Metastable Impact Electron Spectroscopy (MIES). MIES/UPS spectra for the interaction of carboxylic acids (formic and acetic acid) with ASW have been presented and discussed at length previously.7,17 Both the preparation and the annealing of the studied films and interfaces took place under in situ control of MIES and UPS (HeI and II). The MIES results are not presented here, but are available as Supporting Information (see also ref 23). Here, we confine ourselves to a summary of the findings important for the considerations in the following sections: (1) AA species that are adsorbed at 124 K on top of ASW films do not penetrate into them. (2) D2O molecules offered to an AA film, kept at 124 K, become embedded into the AA film. In fact, spectral features from AA remain noticeable in MIES at D2O exposures equivalent to two bilayers (18 L) of D2O. (3) The film formed at 124 K by codeposition of AA and D2O on Ag is terminated by AA species.

Bahr and Kempter (4) Most of the AA species desorb practically together with the water around 160 K for the studied situations (1)-(3). This is expected because the binding energy of the most stable AA monohydrate is predicted to be nearly twice that of the water dimer.6 However, a small portion of the AA molecules remains surface-adsorbed up to more than 200 K; these are the species that are chemisorbed on the silver substrate. (5) From the onset of the spectra at large binding energies, the change of the work function (WF), taking place during the film formation, can be determined: a decrease of WF is observed for AA and D2O deposition on Ag (0.7 and 0.5 eV, respectively), indicative for charge transfer from the adsorbates to Ag. Codeposition of AA and D2O on Ag leads also to a WF decrease (0.8 eV). We relate the saturation of the WF decrease for AA/Ag, observed around 12 L, to the completion of the first AA adlayer at this point. Assuming the same sticking coefficients for Ag and ASW substrates, we conclude that also on ASW an exposure of ≈12 L AA is required to supply the equivalent of 1 monolayer (ML) AA. A WF decrease (increase) of about 0.5 eV can be noticed when AA (D2O) is deposited on an D2O (AA) film. We consider this as an experimental artifact; obviously, the electron ejection, taking place during MIES/UPS, cannot be compensated by charge flow from the substrate through the isolating water or AA films which leads to charge-up effects. 3.2. RAIRS. For a convincing identification of the spectral features seen during the AA-D2O interaction, we have studied the vibrational spectra during the deposition of AA (CH3COOH) and AA-d1 (CH3COOD) on Ag, held at 124 K (denoted by AA/Ag) and AA-d1/Ag, respectively); this is followed by the annealing of the films up to 200 K (i.e., until practically all AA has desorbed). The exposure range that was studied corresponds to coverages between zero and about four adlayers. The identification of the main bands seen between 700 and 1900 cm-1 can be made on the basis of previous IR work on AA monomers, cyclic and linear dimers, and crystals.1-4,12 A large variety of spectral features exist, in particular as far as the νCdO region (between 1600 and 1800 cm-1) is concerned. This is because (a) various conformations, monomers, linear and cyclic dimers, and oligomers are present, and (b) these respective species are subject to H/D exchange as a consequence of their interaction with D2O. Moreover, the νCdO features are subject to energetic shifts depending on the fact that (1) features from unhydrated AA species may either be present as physi-/chemisorbed surface species or as bulk species (in multilayers), and (2) the abovementioned species can become hydrated as a consequence of their interaction with D2O. In the figures, we indicate only the νCdO (around 1700 cm-1), νC-O (around 1300 cm-1), and γOH (around 950 cm-1) regions while the detailed analysis of the spectral features seen in these regions is attempted in Table 1. 3.2.1. Acetic Acid on Ag. RAIRS spectra from the deposition of AA on Ag are presented in Figure 2. These results supplement those presented for AA/Cu10 and AA/Au.7 We note that the νCdO band (peaked around 1705 cm-1 for low coverages) is accompanied at all exposures by a sharp νC-O (1303 cm-1) and a γOH band (around 953 cm-1). This allows us to identify the adsorbed species as cyclic dimers2 chemisorbed onto the Ag substrate. In consideration of the metal surface selection rules, the observed weakness of the νC-C band implies that the cyclic dimer is aligned with its C-C bond parallel to the substrate surface. Around 10 L exposure (completion of the first adlayer), a second band develops in the νCdO region around 1724 cm-1 that dominates up to 28 L. We attribute it to physisorbed cyclic dimers forming the second layer. For

MIES and RAIRS Study of Acetic Acid with D2O

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TABLE 1: Assignment of the νCdO and νC-O Stretching and γO-H Twisting Modes of AA and AA-d1 on Ag and in Interaction with D2Oa AA/Ag assignment

Figure 2 (124 K)

Figure 3 (171 K)

AA-d1/Ag Figure 5 (124 K)

AA/D2O

Figure 6 (161 K)

Figure 7 (124 K)

Figure 8 (165 K)

D2O/AA Figure S5b (124 K)

Figure S5b (164 K)

AA+D2O Figure 10 (124 K)

Figure 11 (158 K)

νCdO surface monomer (M) bulk monomer (M) cyclic dimer (C) chemisorbed on Ag

1794

1788 1745

1794

1794 1744

1703

1705

1697

physisorbed

1724/ 1711

1706/ 1693 1731/ 1711

hydrated nonhydrated L1 hydrated L1 hydrated L2 chain (P)

1792

1787(d) 1748(d) 1712(d)/ 1694(d)

1727

1713(d)/ 1693(d) 1728(d)

1730

1712(d)/ 1693(d) 1730(d)

1711 1745

1744

1748(d) 1716 1689

1647/ 1658

1649

1711 1685 1648(d)

1712 1689 1651(d)

1648(d)

νC-O cyclic dimer (C) nonhydrated L1 hydrated L1

1303

1303

1332

1311

1303 1277(d) 1340 -1220 1340 -1220

hydrated L2 chain (P)

1283(nd)/ 1327(d)

1280(nd)/ 1326(d)

1323(d)

1324(d)

1325(d)

γOH cyclic dimer (C) chain (P)

953

958

957

920 γOD

cyclic dimer (C)

168 K (see Figure 3 of ref 10) which we do observe, as a pronounced band, only for the interaction of AA with water (see below). RAIRS spectra from the deposition of AA-d1 on Ag are presented in Figure 5. These results are important for the interpretation of Figures 6-11 where H/D conversion plays an important role. The spectra in the νCdO region are similar to those for AA/Ag only for exposures below about 12 L; for larger exposures, they become complicated by the overlap of the peak attributed to the formation of the third AA layer (1711 cm-1) with a structure located around 1705 cm-1. In ref 4, it was tentatively attributed to the overtone 2νC-C (which appears not entirely convincing considering the fact that the intensity of νC-C (895 cm-1) is rather small under the present conditions). Presently, this overlap forbids a detailed analysis of the νCdO region. On the other hand, inspection of ref 2 reveals that the spectrum between 1450 and 700 cm-1 is characteristic for cyclic dimers. This is underlined by the presence of a strong γOD band (located at 697 cm-1 for AA-d1 2). Figure 6 displays the RAIRS results from the annealing of the AA-d1 film. Judging from the changes observed between 1735 and 1705 cm-1 in the νCdO band, desorption of the AA multilayer sets in above 149 K and is completed around 161 K. Again, annealing leads to a partial conversion of the cyclic dimers (clearly noticeable above 155 K) into chain frag-

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Bahr and Kempter

Figure 2. RAIRS spectra collected during the deposition of 48 L (≈4 ML) of AA on Ag at 124 K.

Figure 3. RAIRS spectra collected during the annealing (124 to 200 K) of an AA film 60 L (≈5 ML) deposited on Ag.

ments; these species disappear from the surface at 172 K. In contrast to AA/Ag, the activity from the chemisorbed species, identifiable between 172 and 186 K around 1700 cm-1, is twinpeaked for AA-d1 (see solid lines) (probably caused by the abovementioned complication with a overtone band). 3.2.2. Vibrational Spectroscopy on Acetic Acid-Water Structures. To get optimum information on the formed AA-water

complexes, we have deposited AA on ASW films, as well as D2O molecules on AA films, both prepared at 124 K. Moreover, we have studied the codeposition of AA and D2O on Ag at 124 K. In all cases. we have annealed the resulting films beyond 200 K. To study details of the H/D exchange between AA and D2O during the annealing process, we have also carried out supplementary measurements with AA-d1.

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Figure 4. Possible structures of CH3COOH in water. Solvation has been omitted except for the monomer (Mw) structures adapted from ref 4.

Figure 7 shows the results for AA/D2O obtained during AA deposition (40 L) on ASW. The corresponding spectra for AA-d1/ASW are included into the Supporting Information (Figure S6). Both preparation and annealing took place under the control of RAIRS and MIES/UPS. The νCdO bands up to about 13 L (past the completion of the first AA layer) are similar to those found for AA/Ag (see Figure 2). In particular, the νCdO region displays two, partially resolved bands situated around 1716 and 1689 cm-1. While the 1689 cm-1 band saturates around 8 L (0.7 ML), and remains visible as a shoulder only, the one at 1716 cm-1 rises indefinitely and shifts to 1730 cm-1. We observe a strong correlation between the growth of the νCdO band around 1689 cm-1 and the decay of the dangling OD band (dOD); while the 1689 cm-1 band rises strongly up to about 7 L, dOD disappears completely in this exposure region. The twisting mode of the cyclic dimer, γOH (958 cm-1), appears at a rather late stage (around 15 L exposure) and in contrast to AA/Ag remains rather weak. Moreover, the sharp νC-O peak, characteristic for cyclic dimers, does also not appear before the completion of the first adlayer. This suggests that the spectra up to this stage are not characteristic for cyclic dimers (even though the 1716 cm-1 peak, attributed to these species in ref 10 is already clearly seen). The codeposition results, to be discussed below, will provide additional information on this issue. With the growth of the second layer, the spectra become compatible with those from cyclic dimers (see Figure 2). However, the stretching mode νCdO is represented by a broad, intense band centered around 1716 cm-1, while for AA on Ag, starting with the third layer, it consists of two bands around 1724 and 1711 cm-1. In addition, we notice that the activity of γOH is weaker than for AA/Ag by a factor of 4. This suggests that in the studied thickness range the multilayer possesses a different morphology, probably because the first AA layer is different in both cases (see Section 4). The spectra up to about one layer of AA are very similar to those for deposition on ASW adsorbed on Cu.10 It was concluded that the band at 1695 cm-1 corresponds to the formation of an AA-water complex (whose identity could not

Figure 5. RAIRS spectra collected during the deposition of 47 L (≈4 ML) of AA-d1 on Ag at 124 K.

be established, though) with the OH dangling bonds of the water film.10 On the other hand, the band at 1717 cm-1 was considered to be characteristic for cyclic AA dimers. Our observations, in particular the behavior of γOH (not studied in ref 10), suggest that this interpretation may be oversimplified. Figure 8 displays the spectra obtained during annealing (about four layers AA on ASW) (a waterfall diagram with a larger

11308 J. Phys. Chem. C, Vol. 111, No. 30, 2007

Figure 6. RAIRS spectra collected during the annealing of the film prepared in Figure 5.

offset of the spectra is provided in the Supporting Information (Figure S7)). Beyond 140 K, efficient H/D conversion takes place. This manifests itself particularly clearly in the disappearance of the γOH band (around 958 cm-1) and the changes in the νC-O region (1250 to 1350 cm-1). As the inspection of Figures 5 and 6 shows, the bands of the AA species are entirely replaced by the corresponding ones of AA-d1 (including those for the chainlike structure, characterized by the bands at 1648;

Bahr and Kempter

Figure 7. RAIRS spectra collected during the deposition of 40 L (≈4 ML) AA on ASW (film thickness equivalent to 3 bilayers) at 124 K.

1323 cm-1 for AA-d1 3) above 148 K. Efficient H/D exchange was also noticed above 136 K in the TOF-SIMS data for AA deposition on D2O ice films.16 Most of the AA species (those responsible for the 1724 cm-1 band as well as for the shoulder between 1700 and 1620 cm-1) desorb simultaneously with D2O (complete at 161 K). The large probability for simultaneous desorption is expected considering the fact that the AA-D2O

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J. Phys. Chem. C, Vol. 111, No. 30, 2007 11309

Figure 8. RAIRS spectra collected during the annealing (124 to 200 K) of the film prepared in Figure 7.

binding energy is almost twice as large as that of a water dimer.6 The twin-peaked structure (see solid lines), attributed in Section 3.2.1 to chemisorbed cyclic AA-d1 dimers, remains visible up to 182 K. There are no hints for acetate formation. Figure 9 compares the νCdO region from the anneal of ASW films after the deposition of 10, 20, and 40 L AA (about 1, 2, and 4 monolayers). We notice that for 10 L most of the AA

Figure 9. RAIRS spectra (νCdO region) collected during the annealing of AA layers of different thicknesses (10 L (top); 20 L (middle); and 40 L (bottom)) deposited on ASW (film thickness equivalent to 3 bilayers) at 124 K.

species desorb simultaneously with the water (except those becoming chemisorbed on Ag). For 20 and 40 L, a larger portion of AA species survives the water desorption and a variety of

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Bahr and Kempter

Figure 10. RAIRS spectra collected during the codeposition (at 124 K) of D2O and AA molecules (ratio 1:1) on Ag.

Figure 11. RAIRS spectra collected during the annealing (124 to 200 K) of the mixed AA-D2O film of Figure 10.

unhydrated AA species can be recognized. At present, the complicated spectral changes, in particular the strong exposure dependence of the 1746 cm-1 band, are not fully understood. However, based on the results of Section 3.2.1, the following statements can be made. (1) Chain fragments (of the type (P) shown in Figure 4), characterized by the bands around 1650 and 1320 cm -1 for

AA-d1,3 appear during the de-hydration upon annealing. A small contribution to the band at 1748 cm-1 results from the final groups of the chain fragments. However, the overwhelming part of the band, remarkably intense for 40 L AA/D2O, must have a different origin (see (3) below). (2) The weak activity between about 172 and 182 K (see solid lines in Figures 6, 8, and 9) can be attributed to

MIES and RAIRS Study of Acetic Acid with D2O chemisorbed cyclic AA-d1 dimers. Activity from these chemisorbed species can be seen clearly above 170 K (i.e., as soon as most of the hydrated physisorbed AA species have desorbed). (3) Following ref 4, we propose that the 1748 cm-1 peak originates from the weakly distorted CdO group in the components L1, L1′, and L1′′ of various linear dimers (see Figure 4). It has been suggested that bulk monomers do also contribute to this peak.10 This is supported by the fact that the 1748 cm-1 peak is accompanied by νC-O at 1277 cm-1 (see Figure 8), the position expected for AA-d1 monomers.1 Clearly, heating AA films, adsorbed on ASW, offers an interesting possibility to prepare various AA species, ranging from monomers to chain fragments, on which vibrational spectroscopy can be performed with high resolution. These species are formed during the interaction of cyclic AA dimers with ASW, either during the dimer deposition or during the film annealing. While most of the species desorb simultaneously with the water molecules, some of them survive water desorption and can be analyzed as unhydrated species. Figure 10 presents the results for codepositing AA and D2O onto Ag at 124 K. The νCdO region is dominated by a structure located between about 1620 and 1740 cm-1, peaked at 1712 and 1689 cm-1. The νCdO activity rises indefinitely with AA supply. Neither a sharp νC-O, expected for cyclic dimers (1303 cm-1) nor a γOH band (953 cm-1) can be noticed. Thus, the codeposition spectra are not characteristic for cyclic dimers. On the other hand, they are rather similar to those of a 5 M aqueous AA solution in which hydrated linear dimer and monomer species are believed to dominate.4 We observe no dOD band,23 indicating that all D2O molecules are fourfold coordinated and fully involved in hydrogen bonds. To arrive at a convincing identification of the νCdO activity displayed in Figure 9, we rely on refs 4, 10, and 11 where the band νCdO is discussed in detail for various hydrated AA species. From Figure 10, we concluded that the interaction between D2O and cyclic AA dimers during codeposition leads to the breakup of the ring structure. We suppose that (as in aqueous solutions4) various linear dimers of type L, L′, and L′′ (see Figure 4) result from the breakup. Also monomers M may originate as a consequence of the AA-water interaction, as the results for AA/D2O suggest. All these species are assumed to be hydrated. In the following, we estimate their contribution to the νCdO structure. In accordance to refs 4, 6, and 11, the molecules L1, L 1′, and L1′′ and monomers M possess a free CdO group that can accept up to two H-bonds. In accordance to refs 6 and 11, νCdO can experience a red shift of up to 60 cm-1, depending on the type and number of the H-bonds formed. We have concluded in Section 3.2.2 that these species largely contribute to the 1748 cm-1 band, occurring during water desorption. Thus, we suppose that hydration of these species leads to the particular contribution to νCdO that peaks at 1712 cm-1. The νCdO band of those species that are involved in intermolecular bonding, as in dimers for example (CdO groups in L 2, L2′ and L2′′), and thus are strongly distorted, is expected around 1700 cm-1.4 Hydration of these molecules (shown in Figure 4 schematically for the monomer) can then be expected to produce a red shift of up to 30 cm-1.6,11 Thus, these species after hydration may be responsible for the particular activity that peaks at 1689 cm-1. Summarizing, we assign the structures seen in Figure 9 to absorption by CdO groups of hydrated AA species that are (are not) involved in intermolecular bonding (1689 (1712) cm-1). Figure 11 displays the spectral changes observed during the annealing of the film produced by codeposition. Water desorption takes place between 148 and 155 K as can be judged from

J. Phys. Chem. C, Vol. 111, No. 30, 2007 11311 the disappearance of the δOD(D2O) band (1240 cm-1). However, prior to the water desorption, efficient H/D exchange occurs between 136 and 148 K, as judged from the complete disappearance of the νC-O band at 1300 cm-1 (from undeuterated species). Instead, activity centered around 1325 cm-1 occurs. This band is not that of the chain fragments that can be noticed above 155 K as a more narrow one. As discussed above, a variety of deuterated species are present, including chain fragments, as suggested by the band at 1650 cm-1. Above about 163 K, the activity is only due to the chemisorbed species, marked by the solid lines (see Section 3.2.2). Our results for D2O deposition on AA at 124 K can be found as Supporting Information. They can be interpreted as a superposition of the spectra collected from AA multilayers (Figures 2 and 7) and those for codeposition (Figure 10); as compared to codeposition, νCdO and νC-O are more pronounced and considerably sharper and in addition γOH is seen. This indicates that during D2O exposure only part of the AA film (consisting of cyclic dimers) suffers the changes leading to the spectra seen for codeposition, namely, the water-induced breakup of the cyclic dimers, while the rest of the film still consists of intact cyclic dimers. 4. Discussion In this Section we discuss the implications of our results for the mechanisms of the AA-water interaction. Our following interpretation is made in the light of the available theory results.9,10,14,15 At first, we consider the interaction of cyclic AA dimers, offered from the gas phase, with ASW films. We have already concluded in Section 3.2.2 that the initial interaction does not lead to the adsorption of intact cyclic dimers. In accordance to MIES and RAIRS, the AA species adsorb above the ASW film,7 whereby according to our results and those of ref 10 the CdO group of the AA species binds via a hydrogen bond to the dOD present at the ASW surface (also predicted by theory14). The dOD band (2728 cm-1) has disappeared completely at 5 L (less than 0.5 ML coverage), suggesting that at that stage all dOD groups are involved in hydrogen bonds with AA molecules. However, because a broad νC-O band is seen, extending over about 100 cm-1 (Figure 7), it is likely that the C-O-H group of the AA is also involved into the bonding between the AA molecules and the ASW surface. Next, we wish to obtain information on the identity of the adsorbed AA species. For that, we compare the spectra for 5 L exposure (≈0.5 ML coverage) in Figure 7 with those for 12 L exposure in Figure 10 (which correspond nearly to the same AA supply considering the chosen 1:1 mixing ratio). We notice a close correspondence both in the shapes and the intensities of the occurring bands. In Section 3.2.2, we have concluded that codeposition leads to the dissociation of cyclic dimers. Thus, the similarity of the spectra of post AA deposition with those for codeposition suggests that AA adsorption leads to a waterinduced breakup of the dimer rings, and adsorption takes place in form of AA monomers and linear dimers of the type L to L′′ (see Figure 4). This confirms the prediction from MD computations that the adsorption process on ice is able to split cyclic AA dimers at low coverage.9 Moreover, it was predicted on the basis of ab initio and MD calculations that cyclic dimers break up in aqueous environment, leading to water-separated cyclic structures featuring two AA molecules.13 In our previous work,7 we have made a comparison between our MIES/UPS spectra and the density of states of various AAwater complexes, as calculated with DFT at zero temperature.15

11312 J. Phys. Chem. C, Vol. 111, No. 30, 2007 We arrived at the conclusion that AA adsorbs on ASW as cyclic dimers are already in the initial stage of its interaction with ASW (i.e., dissociation does not take place). This conclusion is not supported by the present RAIRS results. As for codeposition (see Section 3.2.2), we attribute the band centered around 1689 cm-1 to hydrated L2 species (see Figure 10) arising from the dissociation of the adsorbed cyclic dimers. This is supported by the finding that for AA in liquid D2O, the band at 1695 cm-1 can be associated with hydrated L2 species.4 The band centered around 1712 cm-1 (see Figure 10) can then, as in Section 3.2.2, be identified with hydrated L1 species. This fits also well with the location of the νCdO band from hydrated monomers (Mw) found at 1711 cm-1.4 These monomers, as the L1 species, possess a carbonyl group that is not involved in intermolecular bonding. According to ref 14, the AA-surface complex formed at the lowest coverage involves an AA monomer and several surface water molecules. The νCdO band of this complex probably contributes to the 1689 cm-1 band, because it is this band that appears at the lowest coverages. The work function (WF) decrease, observed during AA exposure on ASW, continues after the saturation of all dODs, and does not level off before about 12 L, the completion of the first AA adlayer. Up to this stage, the spectra still look rather similar to those for codeposition as far as the shape of the twinpeaked νCdO structure is concerned. This implies that after the saturation of the dODs at 5 L, additional adsorption up to 11 L (about 1 ML) must involve lateral interaction between the AA species of the first layer. It was pointed out on the basis of MD calculations that the lateral interaction during the completion of the first adlayer leads to recombination of AA monomers;9 this may explain the rise of the 1716 cm-1 band at the expense of the band located at 1698 cm-1 during the completion of the first adlayer in Figure 7. The strong increase of the 1716 cm-1 peak during the growth of the second layer, combined with its gradual shift to 1730 cm-1) (whereas the intensity of the 1689 cm-1 band remains fairly constant), suggests that the second layer consists of cyclic dimers. This is underlined by the facts that because (1) the νC-O band develops into a well-defined peak and (2) the γOH band appears at 958 cm-1, both features being typical for cyclic dimers.2 As MIES shows, annealing does not lead to complete solvation of AA; instead, AA remains in the toplayer of the film (i.e., displays surface propensity). From TOF-SIMS results, it was also concluded that AA molecules at monolayer coverage stay at the surface up to water desorption.24 The CH+ 3 intensity in TOF-SIMS remained present up to film desorption, and thus the CH3 group of the AA species is not fully covered by the D2O molecules. In the case of the interaction of salts and strong anorganic acids with liquid and solid water, the surface activity of certain anions has been traced back to their large polarizabilities.25,26 In the present case, however, it appears that the formation of strong hydrogen bonds, involving the carbonyl and hydroxyl groups of AA and surface water molecules, inhibits solvation and keeps AA located at the water film surface. In accordance to MIES, D2O molecules, offered to a film of cyclic AA dimers prepared on Ag at 124 K, become embedded into the AA film. RAIRS proves that an efficient hydration takes place under these conditions; the νCdO bands, attributed to dissociation fragments (1685 and 1711 cm-1), are more pronounced than for AA/ASW. The efficient hydration is also underlined by the smaller number of monomers that are detected during the dehydration when annealing. In accordance to MIES,

Bahr and Kempter the AA species, although hydrated, possess a surface propensity (i.e., even at an exposure of 20 L D2O the AA molecules account for about 20% of the surface area). 5. Summary The interaction of AA with D2O on polycrystalline Ag was investigated. We have prepared acid/water interfaces at 124 K, namely acid layers on thin films of ASW, D2O adlayers on thin acid films, and mixed films from the codeposition of D2O and AA; they were annealed up to 200 K. MIES was utilized to obtain information on the electronic structure of the outermost surface from the study of the electron emission from the weakest-bound molecular orbitals of the acid and the molecular water. RAIRS provided information on the identification of the AA species, their hydration, and the water-induced fragmentation of cyclic dimers, as well as on the water and acid crystallization. On polycrystalline Ag, the first AA layer chemisorbs as cyclic dimers; this is followed by the physisorption of additional layers of cyclic dimers. Acetate formation as a consequence of the AA-Ag interaction is not observed. According to MIES, the AA species (cyclic dimers mainly) offered to ASW films adsorb on top of the solid water films; most of them desorb together with the D2O molecules without becoming solvated. In accordance to RAIRS, the AA species saturate the dangling OD groups at the ASW surface around 50% of a monolayer. Within the first adlayer, the cyclic dimers become dissociated under the influence of their interaction with the D2O molecules. Between 0.5 ML and the completion of the first monolayer lateral interaction between the AA species in the first layer is important but does not lead to the formation of cyclic dimers. The second and following physisorbed layers consist of cyclic dimers. In accordance to both MIES and RAIRS, D2O molecules offered to a film of cyclic AA dimers, prepared at 124 K, penetrate into the film, thereby transforming the cyclic dimer structure of the toplayers into monomers and linear dimers. The same happens for codeposition of AA and D2O at 124 K on Ag. We do not observe water-induced acetate formation under the studied conditions. The heating of AA-D2O films, prepared in the ways described above, offers an interesting possibility for performing vibrational spectroscopy studies on various AA species, either formed during the preparation procedure or during the water depletion; linear dimers, monomers, and fragments of the chains found in crystalline AA become accessible to RAIRS during the late stage of D2O depletion. The vibrational bands of these species are characterized in this work. Acknowledgment. We thank W. Daum (Clausthal) for allowing us to use his RAIRS equipment for the present study. Supporting Information Available: The MIES spectra obtained during the preparation and the annealing of the acetic acid-D2O interfaces as well as the RAIRS spectra obtained during the deposition of D2O on acetic acid films are included as Supporting Information. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Haurie, M.; Novak, A. J. Chim. Phys. 1965, 62, 138. (2) Haurie, M.; Novak, A. J. Chim. Phys. 1965, 62, 147. (3) Haurie, M.; Novak, A. Spectrochimica Acta 1965, 21, 1217.

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