  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 \textit{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 semi-classical
forced oscillator model is constructed, using the same friction coefficient
description of the energy loss, 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. \textbf{82}, 446 (1999)] for these
atoms incident on the surface of a Schottky diode.
