  Non-adiabaticity in adsorption on metal surfaces gives rise to a number of
measurable effects, such as chemicurrents and exo-electron emission. Here we
present a quantitative theory of chemicurrents on the basis of ground-state
density-functional theory (DFT) calculations of the effective electronic
potential and the Kohn-Sham band structure. Excitation probabilities are
calculated both for electron-hole pairs and for electrons and holes separately
from first-order time-dependent perturbation theory. This is accomplished by
evaluating the matrix elements (between Kohn-Sham states) of the rate of change
of the effective electronic potential between subsequent (static) DFT
calculations. Our approach is related to the theory of electronic friction, but
allows for direct access to the excitation spectra. The method is applied to
adsorption of atomic hydrogen isotopes on the Al(111) surface. The results are
compatible with the available experimental data (for noble metal surfaces); in
particular, the observed isotope effect in H versus D adsorption is described
by the present theory. Moreover, the results are in qualitative agreement with
computationally elaborate calculations of the full dynamics within
time-dependent density-functional theory, with the notable exception of effects
due to the spin dynamics. Being a perturbational approach, the method proposed
here is simple enough to be applied to a wide class of adsorbates and surfaces,
while at the same time allowing us to extract system-specific information.
