Electron-hole-pair damping of an excited state of a molecule absorbed on a metal surface

Abstract

We present a theory of the lifetime of a vibrational excited state of a molecule absorbed on a metal surface due to creation of electron-hole pairs at the surface. Our formulation treats the dynamical response of the electron gas at the surface exactly in the random-phase approximation. The Lang-Kohn electron density profile at the surface is used. The molecule is modeled as a two-level system whose dynamic dipole moment is enhanced relative to its gas-phase value by the adsorption process. Explicit results are given for the C—O stretch vibration for CO adsorbed on Cu(100). The calculated value of the lifetime accounts for the order of magnitude of the experimental lifetime. A comparison is made with previous theoretical work. We consider the dependence of the transition rate on the diffusivity of the electron density profile at the surface by comparing the transition rates obtained with use of the Lang-Kohn and infinite-barrier-model profiles.