Shift and broadening of the vibrational mode for a diatomic molecule adsorbed on a metal surface

Abstract

The shift and broadening of the vibrational frequency of a diatomic molecule adsorbed on a metal surface are calculated on the basis of electromagnetic interactions. Finite molecular size and nonlocal metal response are included in a calculation of the response function, whose pole is the complex normal mode frequency. Finite molecular size is described by a model of a point-like vibrational mode within a polarizable sphere. Nonlocal metal response is described by a semiclassical infinite barrier model with a Lindhard dielectric constant generalized to include core polarization and finite electron lifetime. When applied to the case of CO adsorbed on Cu and if the molecule is positioned so as to fit the observed width, it is found the predicted shift is a factor of three or more greater than that observed.