Metastable helium spectroscopy on simple metals: Comparison between low and high work function substrates

In this work we investigate the deexcitation process of a slow metastable helium atom (He*) in its triplet state on simple metal surfaces. We compute the electronic properties of the interacting He*-metal (jellium-like) system, using the embedding Green's function method, within the framework of density functional theory. The Auger deexcitation rates are calculated at different distances from the surface using the Fermi golden rule, and the energy distribution of the emitted electrons is obtained by integrating the Auger rates along the classical trajectory followed by the atom in the scattering process. We present the results for three metal surfaces (Al, Na, and Cs) which are sufficiently different to span the distinct deexcitation mechanisms taking place when the He* interacts with the surface: resonant ionization + Auger neutralization for high work function substrates, and Auger deexcitation for low work function ones. Our results for the deexcitation spectral profiles are in good agreement with the experimental ones.