Helium 1s photoemission and photon stimulated desorption of He⁺ ions by double excitations from adsorbed helium layers: Zero-point motion and matrix effects

Excited with p-polarized light, the near-edge He 1s photoemission signal from monolayers of He adsorbed on the close-packed surfaces of silver, copper, ruthenium, and platinum shows periodic splitting with photoelectron momentum. By applying a simple single scattering model, we explain this effect by zero-point motion induced variation of the photo hole’s image charge screening and interference of the photoelectron’s final state wavefunction. Relativistic one-step photoemission calculations support this interpretation. In the second part of our study, we investigate neutral double excitations of He bilayers adsorbed on platinum by monitoring the emission of He⁺ ions. We identify strong matrix and zero-point motion effects, namely resonances absent for isolated He, density and light polarization dependence, line broadening and a correlation of excitation and kinetic ion-energy.