Chemical shifts and coupling interactions for the bonding vibrational modes for CO/Cu(111) and (100) surfaces

Low-frequency infrared reflection absorption spectra have been measured for isotopic mixtures of CO and ${}_{}{}^{13}{}_{}{}^{}\mathrm{C}$ ${}_{}{}^{18}{}_{}{}^{}\mathrm{O}$ on Cu(100) and (111) surfaces. Although the integrated intensities are only 1/100 of those for the intramolecular modes, both the carbon-metal and the dipole-forbidden hindered rotational modes can clearly be seen in the spectral range 250–350 ${\mathrm{cm}}^{\mathrm{-}1}$ for both isotopes in the admixtures. The data thus allow chemical shifts to be distinguished from coupling effects for these two bonding modes. For the two surfaces, the normalized chemical frequency shifts for the C-O stretch are very similar. However, dramatic differences are observed for the modes directly related to the bonding, i.e., the C-metal stretch and the hindered rotation. The dominant coupling mechanism for the perpendicular modes is shown to be dipole-dipole coupling as confirmed by a comparison to coherent potential approximation calculations. For the parallel modes the data cannot be explained in terms of dipole-dipole coupling, and we postulate that the surface state present on the (111) surface is largely responsible for the observed behavior. When the data are interpreted using a model assuming point dipoles, then the vibrational polarizability can be determined for these modes. A more stringent test of the theory is also provided through estimates of the polarizability obtained from intensities of the carbon-metal stretch mode.