Kinetic electron emission from clean polycrystalline gold induced by impact of slow C+, N+, O+, Ne+, Xe+, and Au+ ions

Ion-induced kinetic electron emission is commonly attributed to collisions of an energetic projectile with quasifree electrons, and to the promotion of atomic levels in binary collisions of the projectile with atomic particles in the solid. The contribution of the promotion processes to the electron emission has been estimated theoretically for all studied systems from molecular-orbital correlation diagrams. As quasifree electron collisional excitations have a sharp threshold at relatively high velocities of the projectiles, their contribution to the electron emission at lower impact velocities should be negligible. We will show, however, that the partial localization of the quasifree electrons due to the presence of the solid surface "washes out" this sharp threshold. This can lead to one-electron excitations at low impact velocities that may be more significant than excitations due to promotion. At the lowest impact velocities the electron emission yields conspicuously level off in some studied cases. Such behavior cannot be reconciled with any existing one-electron model (including the one proposed here), as they all predict a rapid decrease of the electron emission with decreasing impact velocity. In this paper we interpret the leveling-off of the yield in terms of a many-electron excitation mechanism, based on the assumption of spatial and temporal localizations of electronic excitation in the impact zone. The models discussed in this paper will be compared with experimental data on kinetic electron emission from polycrystalline gold bombarded by C+, N+, O+, Ne+, Ne-0. Xe+, and Au+, with kinetic energies below similar to 15 keV, and perpendicular incidence on the surface.