Resonance, Auger, and autoionization processes involving He+(2s) and He++ near solid surfaces

Electronic transition processes of a resonance and of an Auger type which can occur at a solid surface have been studied for incident metastably excited He⁺(2s) and doubly charged He⁺⁺ ions. Atomically clean Ni(100) and Ni(110) surfaces have been used as well as these surfaces with a c(2×2)Se structure adsorbed upon them. It is shown that the principal process of deexcitation of the He⁺(2s) ion involves resonance tunneling to a doubly excited ${He}^{0**}$ state of the He atom followed by its autoionization to the ground-state ion He⁺(1s) with the ejection of a fast electron. Autoionized electrons produced near the surface are 0.7-0.9 eV faster than those produced in free space. The change in the work function from 4.7 eV for Ni(110) to 5.1 eV for Ni(100) reduces the number of peaks of ejected electrons from two to one. These experimental facts are shown to be self-consistent and to arise from energy-level shifts near the solid surface. Several aspects of the kinetics of the two-stage processes in competition with possible single-stage processes are discussed. It is also demonstrated that He⁺⁺ is first resonance neutralized by one electronic charge to the He⁺(2s) state, which process is then followed, closer to the surface, by those observed for incident He⁺(2s).