11112
J. Phys. Chem. B 1997, 101, 11112-11118
Methyl Formate on Ag(111). 1. Thermal Adsorption-Desorption Characteristics and Alignment in Monolayers A. L. Schwaner, Jeffrey E. Fieberg, and J. M. White* Center for Materials Chemistry, Department of Chemistry and Biochemistry, UniVersity of Texas, Austin, Texas 78712 ReceiVed: May 16, 1997; In Final Form: October 13, 1997X
On Ag(111), there is no evidence that methyl formate dosed at or below 120 K dissociates during adsorption and subsequent thermal desorption. Temperature-programmed desorption distinguishes monolayer (145 K) and multilayer (135 K) HCOOCH3 and DCOOCD3 desorption peaks, indicating a weak adsorbate-substrate interaction (ca. 37.4 kJ mol-1 desorption activation energy) that is only slightly stronger than the adsorbateadsorbate interaction. The relative intensities of vibrational bands detected in reflection-absorption infrared spectra vary significantly with coverage and are consistent with adsorption in a thermodynamically stable cis form. HCOOCH3 and DCOOCD3 spectra for monolayer coverages show no modes with a′′ symmetry, as expected when the molecular symmetry plane lies perpendicular to the Ag(111) surface. The relative intensities of modes with a′ symmetry indicate that the carbonyl (CdO) axis is tilted from the surface plane, the CH3 group points toward the substrate, and the C-O bond of the interior ester linkage is nearly parallel to the substrate. Above ML coverage, a′′ modes begin to appear, and for 5 layers or more, the vibrational frequencies are similar to those for gas-phase and matrix-isolated spectra.
1. Introduction From our ongoing study of thermal and nonthermal activation of adsorbed heteroatom hydrocarbon molecules, we report on the alignment and thermal chemistry of the simplest carbonacid ester, methyl formate (HCOOCH3), adsorbed on a relatively inert metal substrate, Ag(111). Temperature-programmed desorption (TPD) mass spectrometry, reflection-absorption infrared spectroscopy (RAIRS), X-ray photoelectron spectroscopy (XPS), work function change (∆Φ), and ultraviolet photoelectron spectroscopy (UPS) were utilized. Methyl formate, frequently involved in catalyzed partial oxidation reactions, is a byproduct of the catalytic formation of formaldehyde and the dehydrogenation of methanol over Cu1 and Ag2,3 substrates. It appears in the decomposition of methanol on oxygen precovered Ag(110),2 but not in the dehydrogenation of methanol or the dimerization of formaldehyde on Cu(110).4 On Ni(111),5 low coverages (10 eV), the sample was rotated 90° away from lineof-sight of the QMS during TPD. For TPD, the heating rate was 1.5 K/s between ∼100 and 690 K. Absolute coverages (molecules/cm2) were calculated from the C(1s) and/or O(1s) XPS peak areas using a standard and relative elemental sensitivities established for our instrument. The standard is 4.6 × 1014 cm-2 of atomic iodine in a readily established x3 × x3R30° structure formed on Ag(111).36 In our experiments this structure was formed by thermal dissociation of CF3I at 500 K to preclude CF3 adsorption and retain the atomic iodine.26b The relative elemental sensitivities came from a literature tabulation.37
J. Phys. Chem. B, Vol. 101, No. 51, 1997 11113
Figure 1. TPD of methyl formate for various dose times (s) after increasing the pressure (see text) by ∆P ) 1 × 10-9 Torr: (a) 35, (b) 40, (c) 45, (d) 60, and (e) 120 s. Curve f is a 45 s dose onto an Ar+ sputtered, but not annealed, surface. The heating rate was 1.5 K/s.
3. Results 3.1. Thermal Desorption. TPD spectra were taken in both systems described above and involve separate Ag(111) substrates. Using the first system, the 60 amu TPD spectra as a function of increasing HCOOCH3 exposure are shown in Figure 1. The spectra possess the ion fragmentation pattern of gasphase methyl formate.35 These results indicate that HCOOCH3 adsorbs and desorbs without dissociation. Data were taken to 500 K, but since all signals drop to background levels at about 180 K, we show only the 100-200 K range. The lowest exposure, curve a, yields a single peak at 145 K, with a shoulder on the high-temperature side. As exposure increases, the 145 K peak intensifies and saturates at 60 s (curve d). For longer dose times, a new peak appears at 133 K (curve e) that does not saturate (not shown) and is ascribed to multilayer desorption, as is the case for Cu(110)4 and Ni(111).5 We define one monolayer of methyl formate in terms of the integrated area of curve d (i.e., the 145 K peak is saturated, and there is no multilayer peak). The total peak area (not shown) increases linearly with dose time, indicating no significant change in the sticking coefficient for adsorption on Ag or on the HCOOCH3 ML. Curve f, which is broad and peaks near 160 K, was obtained after sputtering at 110 K with Ar+ but not annealing. The dose
Figure 2. TPD of DCOOCD3 for several coverages: (a) 0.4, (b) 1, (c) 4, and (d) 20 ML. The heating rate was 1.5 K s-1. The darkened portion of curve a is attributed to adsorption on defect sites (see text).
was equal to that used to obtain curve c, and curves c and f have equal peak areas. Compared to the annealed surface, the key result is a shift to higher temperatures without introducing changes in either the sticking coefficient or the nondissociative character of the process. From this, we infer that the hightemperature shoulder on the other curves is associated with surface defects. Using the second (RAIRS) system, we recorded a few DCOOCD3 TPD spectra for coverages between 0.4 and ∼10 ML (Figure 2). Two peaks for curve C, positioned at 132 and 144 K, correspond to the multilayer and monolayer. As expected, the multilayer peak temperature increases with coverage, reaching 136 K in curve d. The only significant difference between the two Ag(111) substrates is a smaller contribution from defect sites in Figure 2 (darkened area in curve a). Although not shown, HCOOCH3 TPD profiles are indistinguishable from the DCOOCD3 spectra of Figure 2. 3.2. X-ray Photoelectron Spectroscopy. The C(1s) and O(1s) XPS data are presented in Figures 3 and 4, with curves
11114 J. Phys. Chem. B, Vol. 101, No. 51, 1997
Schwaner et al. TABLE 1: Work Function Changes
Figure 3. C(1s) XPS annealing set for HCOOCH3: (a) multilayer at 120 K, (b) 120 K, (c) 150 K, (d) 165 K, and (e) 600 K. Curve b corresponds to a coverage of 1 ML.
Figure 4. O(1s) XPS annealing set for HCOOCH3: (a) multilayer at 120 K, (b) 120 K, (c) 150 K, (d) 165 K, and (e) 600 K. Curve b corresponds to a coverage of 1 ML. These data were taken simultaneously with those in Figure 3.
a and b representing the multilayer and monolayer, respectively. The remaining spectra were taken after heating to a selected temperature and recooling. The two C(1s) peaks evident in the raw multilayer data (dots in Figure 3) were fit with mixed Gaussian-Lorentzian (G-L) functions38 (i.e., 85% G-15% L), resulting in the two solid lines with binding energies (BE) of 287.1 and 289.7 eV and a fwhm of 2.2 eV. The bold line represents the sum of the two. For the monolayer, the intensities are reduced, as expected, and two peaks remain evident; the same fitting procedure yields binding energies of 286.9 and 289.4 eV with fwhm of 2.0 eV. These BE values correspond well to those for molecular HCOOCH3 adsorbed on other surfaces.8-10,39 The 0.2-0.3 eV increase in the multilayer binding energy is attributed to a commonly noted final state effect: the hole left by the ejected electron is less well screened in the presence of multilayer HCOOCH3. Although we anticipate an area ratio of unity for the two types of C in HCOOCH3, we calculate a higher BE:lower BE ratio of 1.08 ( 0.05 for the multilayers and 0.91 ( 0.05 for the monolayer. Using the total C(1s) peak area and the calibration
coverage or T/K
∆Φ/eV
0 ML multilayer 1 ML/120 K 165 K 200 K 400 K
0.0 -1.2 ( 0.2 -0.8 ( 0.1 -0.5 ( 0.1 -0.1 ( 0.1 0.0
described in section 2, we calculate an absolute monolayer coverage of (7.0 ( 0.1) × 1014 molecules cm-2. The O(1s) spectra (Figure 4) exhibit analogous behavior. For multi- and monolayer coverage, the raw data (dots) can be fit with two peaks with fwhm of 2.0 eV. The multilayer binding energies (curve 3a) are 533.2 and 534.7 eV; both decrease 0.4 eV in passing to the monolayer (curve 3b). The higher/lower peak area ratios are 0.88 ( 0.08 and 0.95 ( 0.08 for the multilayer and monolayer, respectively. The uncertainties here, and in the C(1s) ratios, are based on averages over several independent experiments. As expected, heating the surface to 150, 165, and 600 K removes increasing amounts of C and O. Consistent with TPD, there is no evidence for dissociationsthe measured BE’s do not deviate significantly from monolayer values, flashing to 165 K (curves d) removes almost all traces of HCOOCH3, and heating to 600 K leaves a clean Ag(111) surface with neither C or O. The small O(1s) BE drop (0.4 eV) in passing from 1 to 0.4 ML coverage (curve c) may reflect small changes in the final state effects due to reduced crowding in the submonolayer regime. The two discernible binding energies in both C(1s) and O(1s) XPS are readily attributable to the two distinct chemical environments in HCOOCH3. Monolayer 286.9 eV C(1s) BE is assigned to the methyl carbon and the 289.4 eV BE to the formyl carbon. For O(1s), the 534.3 eV BE is assigned to the internal ester oxygen and the 532.8 eV BE to the formyl oxygen. We suppose that the C(1s) sensitivity is too low to allow identification of two peaks in curve d of Figure 2. The multilayer and monolayer peak area ratios for C(1s) and O(1s) are both close to unity, as expected for two equally populated chemical environments that involve no dissociation of HCOOCH3. The ratios, calculated as described, are less than unity, perhaps because the fitting procedure uses fixed fwhm values for both chemical environments and thus fails to account for small variations in structural heterogeneity between the two. 3.3. UPS and Work Function. To broaden the scope of our investigation, we measured ultraviolet photoelectron spectra of adsorbed HCOOCH3 and, from them, determined work function changes (Table 1) and “fingerprint” valence band spectra for 1 ML HCOOCH3 (Figure 5). While details are limited, the spectrum shown (clean Ag(111) subtracted) has peaks at ∼8.0, 9.0, 11.5, and 14.8 eV with profiles consistent with gas-phase UPS spectra40,41 (lines in lower part of Figure 5). In the gas phase, the UPS peak for the nonbonding orbital on the carbonyl oxygen, no, is located at an ionization potential of 11 eV; the next orbital is π2 (11.5 eV), assigned to the antisymmetric, approximately nonbonding orbital associated with the interior O-C-O regime. In order, the deeper gasphase orbitals are σ (13 eV), πme (14.2 eV), σCO (15 eV), πCdO (16.7 eV), and σCO (17.4 eV). Aligning the 8.0 eV peak with the highest lying gas-phase lone pair orbital requires a 3.5 eV shift. The same shift (Figure 5) reasonably aligns the 11.5 eV peak with πme and σCO and the 14.8 eV peak with the πCdO and σCO orbitals. Consistent with reversible adsorption and desorption of HCOOCH3, no evidence for any residual adsorbate appears in UPS spectra after annealing to 200 K or higher.
Methyl Formate on Ag(111). 1
Figure 5. Monolayer HCOOCH3 He(II) UPS difference spectrum. A scaled version of the clean Ag(111) spectrum has been subtracted. The negative portion located near 4.8 eV BE is due to imperfect subtraction of the Ag d band. Gas-phase transitions, uniformly shifted as described in the text, are shown as solid vertical lines at the bottom.
Figure 6. RAIRS for HCOOCH3 adsorbed on Ag(111). Curve a is for 1 ML and curve b for 5 ML.
Work function changes as a function of methyl formate coverage and annealing temperature appear in Table 1. As expected for a weakly held polarizable adsorbate, the work function decreases upon adsorption of HCOOCH3: -1.2 ( 0.3 eV for multilayer and -0.8 ( 0.5 eV for monolayer. Upon subsequent annealing, the work function rises and reaches a clean surface value at 400 K, consistent with the TPD data. On the basis of the monolayer work function change (∆φ) and the monolayer coverage, ns, estimated above (7 × 1014 molecules cm-2), the normal component of the effective dipole moment per adsorbed HCOOCH3 is roughly estimated as 0.3 D (calculated assuming only the normal component, µs, contributes to the work function change ∆φ ) ns‚µs). This rough estimate compares with 1.90 D calculated for isolated cis-HCOOCH3.42 This estimate applies only to monolayer coverage; an estimate is not available for the dipole moment of isolated adsorbed HCOOCH3. 3.4. Reflection Absorption Infrared Spectra. HCOOCH3. Figure 6 displays the RAIR spectra for monolayer (curve a) and multilayer (curve b) coverages. Table 2 summarizes the observed RAIRS modes and the gas,15-17 liquid,17,20 and matrix18 phase modes and their assignments for the cis conformation. Notice that, in some cases, interpretation is complex; strong mixtures (in terms of CH3, HCO and OCO local motions) are involved (last column of Table 2).16
J. Phys. Chem. B, Vol. 101, No. 51, 1997 11115 The RAIR spectrum of multilayer HCOOCH3, Figure 6b, exhibits 10 distinguishable peaks. Those at 1159, 1451, 2959, 2984, and 3015 cm-1 correspond to CH3 vibrational modes, while those at 1677, 1703, and 1733 cm-1 are assigned to CdO stretching vibrations. Peaks at 1175 and 1211 cm-1 are assigned to modes that incorporate stretching of the Ce-O bond (e ) ester carbon and m ) methyl carbon). A shoulder on the highfrequency side of the 1211 cm-1 band that overlaps with the monolayer spectrum is discussed below. Among these multilayer assignments, modes with both a′ and a′′ symmetry make significant contributions (column 6 of Table 2). There are fewer bands in monolayer HCOOCH3 (Figure 6a); five separate peaks appear at 1240, 1446, 1677, 1742, and 2984 cm-1. Compared to the multilayer spectrum, modes at 1159 and 1175 cm-1 are notably absent. The peaks at 1446 and 2984 correspond to CH3 vibrational modes. Both 1677 and 1742 cm-1 peaks lie in the carbonyl, CdO, stretch regime. There is only one carbonyl band for HCOOCH3 in an Ar matrix (1746 cm-1)18 and for neat HCOOCH3 (1725 cm-1).20 In the gas phase,15,16 the carbonyl stretch lies at 1754 cm-1 and has a rotational band contour that leads to intensity maxima both above and below this central position. The remaining peak in curve 6a, 1240 cm-1, which likely contributes as a shoulder in curve b, is assigned to one of the complex modes (ν8 in Table 2) that involves mainly CH3 rocking and Ce-O stretching, with a smaller contribution from Cm-O stretching.18 In the liquid phase,20 this mode appears at 1209 cm-1, consistent with the frequency observed for multilayer HCOOCH3 (Figure 6b). Among these monolayer mode assignments, there are no modes with a′′ symmetry (see Table 2). Further, the relative intensities of the a′ modes, particularly the absence of signal at 1175 cm-1, differ dramatically from the matrix-isolated spectra. Note the interesting frequency shifts between the monolayer and multilayer spectra. The low-frequency carbonyl peak for the monolayer is red-shifted with respect to both the multilayer (∼30 cm-1) and matrix-isolated HCOOCH3 (∼70 cm-1), whereas the higher energy component is shifted no more than 9 cm-1. On the other hand, the 1240 cm-1 peak is blue-shifted by 35 cm-1 with respect to matrix-isolated HCOOCH3. DCOOCD3. RAIR spectra (Figure 7) of fully deuterated methyl formate, DCOOCD3, show similar coverage variations. Several modes are suppressed at low coverages, as is the case for HCOOCH3. For 0.7 ML, there is only one carbonyl mode (1647 cm-1), but for 1 ML, there are two (1651 and 1706 cm-1). These are accompanied by three other modes at 899, 1055, and 2241 cm-1. At 5 ML, two peaks appear in the C-D stretching region at 2236 and 2265 cm-1, and, more significantly, a strong band emerges at 1223 cm-1. For 20 ML, weak bands at 885, 1096, 2084, and 2188 cm-1 can be seen; otherwise the 20 ML spectra differ from the 5 ML in only two notable ways: (1) in the carbonyl stretching region, the intensity of the 1673 cm-1 band grows more rapidly than the 1695 cm-1 band, and (2) in the C-D stretching region, the 2265 cm-1 band overwhelms the 2188 cm-1 band at 20 ML. Along with literature data, Table 3 summarizes our assignments and their approximate descriptions for DCOOCD3. For most bands, the thick (20 ML) multilayer spectra differ very little from matrix-isolated DCOOCD3.18 There is one difference; the carbonyl region for the matrix-isolated spectrum exhibits only one peak (1706 cm-1). Features at 910, 1054, 1062, 2084, and 2265 cm-1 are attributed to CD and CD3 normal modes, and the intense 1225 cm-1 band is assigned to a normal mode involving significant Ce-O stretching. There are other normal modes at 1081 and 1086 cm-1; all the bands in the 1050-1100
11116 J. Phys. Chem. B, Vol. 101, No. 51, 1997
Schwaner et al.
TABLE 2: Vibrational Frequencies of Methyl Formate (HCOOCH3) Adsorbed on Ag(111) Ag(111) 1 MLa
1240
1446 1677 1742
2984 a
Ag(111) >5 MLa
cis-MF-(Ar matrix) (absorbance)18
1159 1175 1211 1240(sh)
1158(14) 1162(77) 1205(100)
1451 1446(sh) 1677 1703 1733 2959 3015 2984
MF-liquid phase17,20
MF-gas phase15-17
mode and symm. species18
approx. description18
1168
(ν15, a′′) (ν9, a′) (ν8, a′)
CH3-d-r′ CH3-d-r -OCO-def. + CeO-str. - Cm-Ostr. CeO-str. -CH3-d-r -CmO-str.
1465
(ν7, a′) (ν6, a′) (ν14, a′′) (ν5, a′)
CH i.p.-wag CH3-s-def. CH3-d-def.′ + CH3-d-r′ CH3-d-def. + CH3-d-r
1209
1372(2) 1434(9) 1446(9) 1459(11)
1381 1434
1746(48)
1728
1754
(ν4, a′)
CdO-str.
2957
2969 3012
(ν3, a′) (ν2, a′) (ν13, a′′) (ν1, a′)
CH-str. CH3-s-str. CH3-d-str.′ CH3-d-str.
2938(19) 2963(11) 3014(5) 3033(3)
1445
This work.
Figure 7. RAIRS for DCOOCD3 adsorbed on Ag(111): (a) 0.7, (b) 1, (c) 5, and (d) 20 ML.
cm-1 region are assigned to modes dominated by symmetric and antisymmetric methyl bending. As for HCOOCH3, no a′′ modes appear at or below 1 ML and there are notable absences among the a′ modes, particularly the 1223 cm-1 mode. Like 1 ML HCOOCH3, the lowfrequency carbonyl mode for 1 ML DCOOCD3 is red-shifted (55 cm-1), the high-frequency carbonyl mode is shifted no more than 10 cm-1, and the 899 cm-1 mode, dominated by the Cm-O stretch, is blue-shifted (60 cm-1) compared to matrix-isolated methyl formate. Finally, we note the absence of intensity in the 1350-1400 cm-1 and/or 1550-1600 cm-1 region for both HCOOCH3 and DCOOCD3. These modes, the symmetric and antisymmetric stretches of a bidentate formate (carboxylate) surface species, were observed when methyl formate was adsorbed on supported Cu catalysts.11-14 Bidentate acetate species were also observed when acetic acid was adsorbed on preoxidized Ag(111).25 Since we observe none of these modes, we surmise that methyl formate undergoes no nucleophilic attack or dissociation upon adsorption at 100 K. 4. Discussion TPD, RAIRS, XPS, UPS, and ∆Φ measurements indicate that the thermal behavior of HCOOCH3 on Ag(111) during adsorption at 120 K and subsequent desorption is nondissociative. This observation serves as important background for the
companion paper, which demonstrates that dissociative chemistry is induced by incident electrons.28 Reflecting the two chemically distinct C and O atoms in HCOOCH3, the monolayer XPS data are readily described by molecular adsorption; there are two distinct BE’s of nearly equal intensity for both C(1s) and O(1s). The UPS data, while not conclusive, are consistent with gas-phase data and are interpretable without invoking any species other than parent. The TPD data support this interpretation and provide further insight. The monolayer and multilayer features appear well below 200 K and are separated by about 10 K. Using the simplest estimates based on peak temperatures and first-order kinetics,43 the 133 and 145 K thermal desorption peaks of Figure 1 give a monolayer desorption activation energy of 37 kJ mol-1, only 10% higher than for multilayer desorption (34 kJ mol-1). This indicates that HCOOCH3 coupling to Ag(111) is weak and only slightly stronger than methyl formate coupling to itself. The weak HCOOCH3-Ag(111) interactions are also consistent with the absolute monolayer coverage estimated from XPSs (7.0 ( 0.1) × 1014 molecules cm-2 (close to 5 × 1014 molecules cm-2, an estimate based on the liquid-phase density (0.89 g cm-3) of HCOOCH3). To ascertain the coverage dependence of the cis-HCOOCH3 orientation, particularly at the monolayer level, we took RAIRS measurements. As outlined below, the data are consistent with a monolayer model in which the symmetry plane of methyl formate is perpendicular to the substrate, the carbonyl and methyl groups are toward the substrate, the CdO bond lies near the surface normal, and the Ce-O bond is nearly parallel to the substrate (Figure 8). In the multilayer regime, the molecules are more randomly oriented. Methyl formate has twelve in-plane (a′) and six out-of-plane (a′′) normal modes and can exist in cis or trans form. As noted, the gas-phase cis isomer (i.e., the double-bonded oxygen is cis to the methyl group, as in Figure 8) is ∼20 kJ/mol more stable than the trans isomer, and there is a 40 kJ/mol21,22 rotational barrier between the two forms. Preparing the trans isomer has proven difficult. Muller et al.,18 after irradiating matrix-isolated cis-HCOOCH3 at 248 nm for several days, were able to convert 50% to the trans configuration. More recently, strong vibrational evidence has been reported for gauche-to-trans ethyl formate conversion accompanying the adsorption of ethyl formate on Ni(111) at 100 K.44 The evidence for cis-to-trans conversion of methyl formate on Ni(111) is less clear.44 Unlike Ni(111), Ag(111) does not dissociatively chemisorb HCOOCH3. Rather, and typical of this substrate, HCOOCH3
Methyl Formate on Ag(111). 1
J. Phys. Chem. B, Vol. 101, No. 51, 1997 11117
TABLE 3: Vibrational Frequencies for Methyl-d4 Formate (DCOOCD3) 0.7 Ma
1 MLa
899
900
1055
1055
5 MLa
∼20 MLa
matrix18
liquid15
908
909 885 904(sh) 1054 1062 1081 1086 1096 1200(sh) 1225 1673 1695 2084 2188 2236 2265
839(18)
867
905(5) 1055(4) 1059(6)
903 1041 1056
1098(39)
904(sh) 1054 1062 1081 1092 1223
1647
2241 a
1651 1706
2241
1672 1696 2236 2265
symmetry18
approx. description18
884
(ν10, a′)
CmO-str. - CD-i.p.-wag + CeO-str.
(ν15, a′′) (ν14, a′′) (ν7, a′) (ν6, a′)
CD3-d,r’ - CD-o.p.-wag CD3-d-def′ CD3-d-def. CD3-s-def. + CmO-str.
1091
908 1056 1060 1086 1090
1199(100)
1199
1210
(ν5, a′)
CeO-str. - OCO-def.
1706(51)
1698 1740 2084 2198 2252 2277
1700-1757
(ν4, a′)
CdO-str. - CeO-str.
(ν3, a′) (ν2, a′) (ν13, a′′) (ν1, a′)
CD3-s-str. CD-str. - CdOstr. CD3-d-str.′ CD3-d-str.
2087(6) 2203(24) 2265(7) 2279(6)
gas15,16
2084 2202 2259 2279
This work.
Figure 8. Schematic of alignment model for monolayer methyl formate adsorbed on Ag(111).
adsorption is characterized as very weak chemisorption (or perhaps strong physisorption) where the adsorbate-substrate and adsorbate-adsorbate attractive potentials are approximately equal. Given these considerations, and since our vibrational data contain no compelling evidence for including the trans isomer, the following discussion assumes trans makes, at most, a minor contribution and that the cis conformer dominates the adsorbed phase at all coverages. Applying the RAIRS selection rule, i.e., only vibrations with a transition dipole moment component perpendicular to the surface are active, allows us to draw some structural conclusions. For molecules oriented with the symmetry plane perpendicular to the surface, modes of a′′ symmetry would be strongly suppressed, while the a′ modes will be active (inactive) if their transition moments have (do not have) components perpendicular to the substrate surface. Since the multilayer RAIR spectra contain contributions from both a′ and a′′ modes, we can safely conclude that multilayer HCOOCH3 orientation distribution includes significant contributions from molecules whose symmetry plane is tilted with respect to the surface normal. Further, since monolayer and submonolayer coverages contain no detectable contributions from a′′ modes, we conclude that the symmetry plane of these molecules lies perpendicular to the Ag(111) substrate. In the monolayer, the carbonyl groups are likely oriented toward the substrate, where oxygen lone pair interactions with the substrate are maximized. This orientation is consistent with the red-shifted carbonyl modes as observed when Lewis acids (e.g., BF3) interact with HCOOCH3.23 The presence of at least two carbonyl modes for all coverages, except for the 0.7 ML DCOOCD3 spectra, is attributable to different sources, depending on the coverage. For the monolayer coverage, we propose two distinct adsorption sites of the molecule with respect to the substrate. The first, preferentially populated at coverages up to 0.7 ML for DCOOCD3, has the largest carbonyl red-shift,
i.e., the 1677 cm-1 band for HCOOCH3 and 1651 cm-1 for DCOOCD3, and the strongest coupling to Ag(111). We note that the molecular density in the 0.7 ML coverage (0.7 × 5 × 1014 molecules cm-2) is equivalent to the extrapolated liquidphase density of 5 × 1014 molecules cm-2 (see above calculation based on XPS intensities). Between 0.7 and 1 ML (i.e., the adsorbate density exceeds the liquid phase density), the added (and probably some of the existing) adsorbates occupy another site altogether. This site is occupied by methyl formate molecules that have the same orientation as those for the e0.7 ML coverage (since no new RAIRS bands are observed) but with less overlap between the carbonyl oxygen and the substrate. This accounts for the higher frequency carbonyl mode, which is red-shifted very little with respect to the matrix-isolated case.18 Interadsorbate coupling, which splits the carbonyl band, may contribute in the multilayer regime. The lower frequency component in the multilayer differs significantly from the lowfrequency monolayer carbonyl band, particularly in the HCOOCH3 spectra. We note that multiple red-shifted CdO stretching modes are observed between 1650 and 1730 cm-1 when ethyl formate interacts with Sb/Cl-based Lewis acid adducts.23 Finally, a single red-shifted carbonyl stretch is observed at low (
