Energy Loss of Atoms at Metal Surfaces due to Electron-Hole Pair Excitations: First-Principles Theory of “Chemicurrents”

A method is presented for calculating electron-hole pair excitation due to an incident atom or molecule interacting with a metal surface. Energy loss is described using an ab initio approach that obtains a position-dependent friction coefficient for an adsorbate moving near a metal surface from a total energy pseudopotential calculation. A semiclassical forced oscillator model is constructed to describe excitation of the electron gas due to the incident molecule. This approach is applied to H and D atoms incident on a Cu(111) surface, and we obtain theoretical estimates of the "chemicurrents" measured by Nienhaus et al. Phys. Rev. Lett. 82, 446 (1999) for these atoms incident on the surface of a Schottky diode.