Molecular dipole model for the internal optic modes of surface adsorbed molecules: Classical image theory

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Abstract
A molecular dipole model is developed to describe dipolar coupling in surface adsorbed molecules using classical image theory. Only the internal optic modes of the adsorbed molecules are considered. Expanding the dipoles associated with both real and image molecules in terms of the same molecular normal coordinate results in a frequency shift twice that previously expected for dipolar coupling in adsorbed molecules. Application of the model to carbon monoxide adsorbed on platinum indicates that the observed frequency versus coverage behavior can be attributed to dipolar coupling. The irreducible corepresentations of Shubnikov type II, or grey, groups are used to describe the symmetry and spectroscopic activity of the molecule‐plus‐image system. Infrared activity is associated only with molecular vibrations having nonzero dipole moment derivative components perpendicular to the surface. Raman activity is associated only with those molecular vibrations having nonzero Raman scattering tensor elements αxx, αyy, αzz, or αxy, where z is perpendicular to the surface.