Intra- and inter-atomic auger processes in collisions of low energy He++, He+, He* (23,S, 21 S) and Li+ with Li covered W(110) surfaces

Electron spectra from He⁺⁺, He⁺ and Li⁺ (10 to 1500 eV) ions colliding under grazing incidence with Li covered W (110) surfaces are reported. The results are compared with those obtained from thermal collisions of (2³ S; 2¹ S) metastable He atoms. In these collisions 1s vacancies are either produced during the collision event (energetic He⁺ (Li ⁺) collisions) or are brought into the collision (slow He⁺⁺ (He⁺, He*) collisions). Population of the 2s orbitals by two electrons produces states which decay by intraatomic Auger processes: we observe autoionization of He** (2s ²) and Li** (1s 2s ²) as well as autodetachment of He^?* (1s 2s ²). Alternatively the 1s-holes in the projectile or target (Li) can be filled by Auger processes involving one or two surface electrons. The processes leading to electron emission are studied as a function of the Li coverage in the submonolayer region (0≦Θ_Li≦1Ml) and as a function of the projectile energy. It is concluded that with one or two 1s vacancies present in the projectile the double capture of two surface electrons constitutes an important process responsible for electron emission of low work function surfaces.