H- formation in grazing collisions of fast protons with an Al(111) surface

We have investigated fractions of negative hydrogen atoms after the interaction of fast protons with energies ranging from 1-30 keV with a clean Al(111) surface under grazing angles of incidence. The observed H- fractions are generally small (<0.5%) and depend strongly on the angle of emergence and the state of preparation of the target surface. We find a resonance type of dependence for the negative ion yield on projectile velocity component parallel to the surface plane, which can be understood in a model of resonant one-electron tunneling between surface and atom. This model implies that H- formation in grazing surface scattering proceeds in a two-step process of single-electron transfer. (orig.)