Inelastic Electron Scattering by a Collective Vibrational Mode of Adsorbed CO

The C-O stretching vibrational mode of the Cu(100) + $c(2\mathrm{}\times 2) \mathrm{CO}$ system has been examined by angle-dependent inelastic electron scattering. The mode is collective and its dispersion is found to be dominated by dipole-dipole interactions among the adsorbed molecules. The vibrational excitation cross section is satisfactorily described by dipole scattering theory.