STUDY OF H- FORMATION IN GRAZING SURFACE COLLISIONS

By combining the transfer Hamiltonian (TH) formalism with a rate-equation approach we have studied the final formation of negative hydrogen ions in grazing scattering of fast protons at the surface of a free-electron metal. In particular, the influence of the image potential on the dynamics of H- formation has been investigated. Good overall agreement of our model calculations with recent experimental data for an Al(111) surface is found. Furthermore, we have derived a simple analytical relation of the H- fraction in the scattered beam with the particle velocity parallel to the surface, the surface work function and the effective binding energy of the H- ion, respectively. This formula may be used as a tool to analyze experimental data. In particular, it allows a reliable estimate of the effective atom-surface distance of final H- formation along the outgoing path of the projectile in grazing ion-surface scattering experiments. The sensitivity of resonant transfer rates and H- formation probabilities, respectively, on the shape of the surface-potential barrier is also studied.