Spin polarized metastable helium de-excitation processes on metal surfaces

Spin polarized de-excitation of a metastable helium atom interacting with metal surfaces is treated within density functional theory. The method is based on a self-consistent calculation of the spin dependent electronic properties of the system, such as the surface density of states and the localized surface states, to compute the transition rate. On the high work function Ag(100) and Ag(111) surfaces, the helium 2s electron is delocalized in the metal and hence the transition rate is weakly spin dependent. The existence of a Shockley surface state in Ag(111) determines a neutralization rate that is about 59% larger than that from Ag(100). On a low work function metal, namely Na(100), the rate is of smaller magnitude than those on silver because the 2s triplet resonance is found to be more occupied. Consequently, emitted electrons can display a strong spin dependence also for a paramagnetic surface.