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

Abstract

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 ${\text{H}}^{+}\text{-Al}$ 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 $\text{H}(1s)$ fractions in the velocity range $0.1\ensuremath{\le}v\ensuremath{\le}2\text{ }\text{a}\text{.u}\text{.}$ for protons impinging on Al(111) with an angle of incidence of $0.50\ifmmode^\circ\else\textdegree\fi{}$. 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 $v\ensuremath{\le}1.5\text{ }\text{a}\text{.u}\text{.}$ although the surface-plasmon-assisted capture mechanism has a significant contribution around the Fermi velocity $({v}_{F}\ensuremath{\simeq}0.9\text{ }\text{a}\text{.u}\text{.})$. For higher velocities, Auger processes mainly explain the experimental findings.