Threshold effects in surface-plasmon transition rates and neutral fractions for protons traveling near a metallic surface

In this work the ion's velocity dependence of surface-plasmon-induced transition rates for proton neutralization at metal surfaces is analyzed. In particular we study the opening of the collective channel for the H+-Al system, which is known to be closed in the fixed ion approximation. The threshold velocity of the collective rates, as a function of the surface-plasmon wave vector and the atomic energy shift, is also analyzed. Afterward, we have computed outgoing H(1s) fractions in the velocity range 0.1 < v < 2 a.u. for protons impinging on Al(111) with an angle of incidence of 0.50 degrees. Capture and loss for resonant and Auger processes are taken into account and the role of the surface-plasmon-assisted electron capture mechanism is investigated. Consideration of all these processes allows us to obtain a good agreement with experimental data. A detailed analysis of our results shows that resonant processes dominate for v < 1.5 a.u. although the surface-plasmon-assisted capture mechanism has a significant contribution around the Fermi velocity (v(F)similar or equal to 0.9 a.u.). For higher velocities, Auger processes mainly explain the experimental findings.