Electronic Damping of Atomic and Molecular Vibrations at Metal Surfaces

Abstract

First-principles calculations of the damping rate of vibrational and translational modes of single hydrogen atoms and molecules chemisorbed on metal surfaces are presented. The decay into electron-hole pair excitations is considered. The metal is described in the so-called jellium model, and the calculations are based on the Kohn-Sham density functional formalism extended to the quasi-static regime. For the actual evaluation of the damping an embedding scheme is used, which through explicit tests is shown to be appropriate. In particular in the homogeneous limit, a comparison is made with an exact phase-shift formula. The local electron density is found to be a key parameter for the damping rate, in particular in situations with no dramatic electron structure at the Fermi level. Adsorbates that induce states around the Fermi level should exhibit an enhanced damping rate. A direct relation between the friction coefficient of an adparticle and the vibrational damping rate is derived. The calculated rate values imply that the electronic mechanism is able to accommodate typical thermal energies of hydrogen atoms impinging on metal surfaces. From this result and comparisons with observed lifetime broadenings of vibrational spectral lines it is concluded that electron-hole pairs provide an important channel for energy transfer at metal surfaces.