Infrared reflection absorption (IRA) spectra were measured for acrolein adsorbed on an evaporated silver film at 90 K under ultrahigh vacuum conditions. IRA spectra indicated that acrolein adsorbed on the silver substrate exists in a trans form and that, on increasing exposure levels, the adsorbate takes on type-A, -B, -C, and -D structures successively. The adsorption structures are characterized by the C-0 stretching (v(C=O)) and CH2 out-of-plane bending (u(CH2)) bands; they are observed at 1670 and 978 cm-' (type-A), 1693 and 985 cm-' (type-B), 1699 and 995 cm-' (type-C), and 1691 and 993 cm-' (type-D). Type-A and -B interact directly with the substrate, while the type-C and -D form molecular layers on top of the type-A and -B layer.The intensity ratios, w(CH~)/Y(C=O), observed for type-A, -B, -C, and -D are appreciably larger than the ratio observed for acrolein in a polycrystalline state, indicating that all the adsorbates have the molecular plane more or less parallel to the substrate surface. In the CH out-of-plane bending vibration region, type-C and -D give the CH out-of-plane bending bands of the vinyl group and formyl group near 1018 and 1005 cm-', respectively, in addition to the u(CH2) band, while type-A and -B give only the prominent band due to w(CH2). This trend was observed also for acrylic acid lying flat on a silver substrate (Fujii, S . ; et al. Sur$ Sci. 1994,306, 381). Thus, the selective enhancement of the u(CH2) band seems to be a general feature for a planar molecule containing a vinyl group and interacting directly with the silver surface. Ab initio molecular orbital method at the RHF/4-3 1G* level was applied to calculate the normal vibrational frequencies of a free acrolein as well as a silver ion-acrolein complex; the results reproduced frequency differences between IRA bands of type-A and the corresponding bands of acrolein in a gas state, proving that type-A forms a coordination bond between the oxygen atom of the C=O group and a positively charged site of the substrate.

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Infrared reflection−absorption spectra in the CH2 out-of-plane wagging (ω(CH2)) vibration region were measured for ethylene (C2H4) adsorbed on Ag(110) as well as on the oxygen-induced p(n×1) reconstructed surfaces of Ag(110) (n = 6, 4, 3, and 2) at 80 K. C2H4 on Ag(110) gives a main peak at 955 cm-1, whereas C2H4 on p(n×1)O−Ag(110) (n = 6, 4, 3) gives rise to a 972−976 cm-1 band (α-state) at low exposures, shifting it to 966−970 cm-1 (β-state) at saturation coverage. The adsorption behavior of C2H4 on the p(n×1) surfaces (n = 6, 4, 3) are explained by assuming that (i) adsorption sites exist between the added Ag−O rows parallel to the 〈001〉 direction; (ii) adsorption sites on both sides of the added Ag−O row form a special pair; (iii) at lower coverages one of the pair is selectively occupied, resulting in the formation of the α state. At higher coverages, where all the sites for the α state are occupied, C2H4 begins to occupy the other site of the pair, forming the β state. Thermal desorption spectra were measured for C2H4 on Ag(110) as well as on the atomic oxygen reconstructed surfaces. The desorption on Ag(110) consists of a state with a peak temperature = 110 K, whereas those on p(n×1)O−Ag(110) (n = 6, 4, 3) consist of two states, corroborating the adsorption model on these surfaces derived from the IR spectra. The desorption temperatures at the α states are found to increase as follows:  130 K (p(6×1)) < 145 K (p(4×1)) < 160 K (p(3×1)), which indicates that the stability of the α states increases with the surface coverage of the atomic oxygen. C2H4 on p(2×1)O−Ag(110) does not take either the α or the β state, but exists in an irregular state, giving a broad feature centered at 970 cm-1 for the ω(CH2) band region. This can be explained by considering that the space between the added Ag−O rows on p(2×1)O−Ag(110) is too narrow to deliver the adsorption sites for the α and β states.

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Infrared spectra were measured at 80 K for dimethyl ether (CH3OCH3, DME) and dimethyl ether-d6 (CD3OCD3, DME-d6) with increasing amounts of exposures to metal substrates, Ag(110), Cu(110), and their atomic oxygen-reconstructed surfaces, p(2 × 1)O−Ag(110) and p(2 × 1)O−Cu(110). At relatively lower surface coverages, the IR spectra of DME on Ag(110) and Cu(110) in the 1500−800 cm-1 region give rise to IR bands mainly ascribable to A1 species, including symmetric COC stretching (νs(COC)) bands at 903 cm-1 on Ag(110) and 895 cm-1 on Cu(110), while at nearly saturation coverages, the adsorbate gives IR bands ascribable to B1 and/or B2 species in addition to the A1 bands with the νs(COC) band discretely sifted to 915 cm-1 on Ag(110) and to 901 cm-1 on Cu(110). Similar distinct spectral changes were observed also for DME and DME-d6 on the reconstructed surfaces. The stepwise spectral changes were interpreted in terms of a conversion from a state of DME with the C 2 axis almost perpendicular to the surfaces to a state with the C 2 axis tilted away from the perpendicular orientation. Fermi resonance effects cause stepwise but complicated spectral changes in the CH3 stretching vibration region of DME during the conversion. The changes strongly depend on the kind of the substrates, in contrast to the spectral changes in the 1500−800 cm-1 region, suggesting that the analyses of Fermi resonances can delineate subtle differences in the DME/substrate interaction modes among the substrates. Density functional theory (DFT) molecular orbital calculations were carried out on the cluster models of DME/Cu(110), where the oxygen atom of DME is coordinated to two copper atoms in the surface of metal clusters consisting of 12 copper atoms in the first layer and six copper atoms in the second layer. The results of calculations reproduce the observed frequencies appreciably well, substantiating the coordination interaction model.