Theory of the resonance ionization of metastable helium on clean and NO-covered Pd(111) surfaces.

Abstract

A theory of resonance ionization (RI) of metastable ${\mathrm{He}}^{\mathrm{*}}$ atoms in interaction with a clean and NO-covered Pd(111) surface is presented which is based on stationary scattering theory. The exact electron scattering states and the RI-inducing potential as well as the adsorption behavior of ${\mathrm{He}}^{\mathrm{*}}$ and NO on Pd(111) are obtained within the same self-consistent quantum-mechanical mean-field approach. The rate of ${\mathrm{He}}^{\mathrm{*}}$ 2s resonance ionization is nearly a factor 35 larger on the NO-covered Pd(111) surface compared to clean Pd(111) at the same perpendicular distance with respect to the first layer of Pd atoms. This is in contrast to earlier expectations that molecular species (NO, CO) adsorbed on transition-metal surfaces should play a shielding role with respect to the RI of metastable ${\mathrm{He}}^{\mathrm{*}}$ atoms. The enhanced rate of RI on NO/Pd(111) is due to the strong direct (electrostatic and quantum-mechanical) and indirect interactions (through the metal surface, including the interactions with the image potential) in the adsorption layer. The broadening of the diffuse unoccupied NO states through coupling to unoccupied metal band states leads to quasidegeneracy with the ${\mathrm{He}}^{\mathrm{*}}$ 2s state, which provides the necessary condition for a resonance process.