Velocity Scaling of Ion Neutralization in Low Energy Ion Scattering

The ion fraction $P^{+}$ is measured for ${\mathrm{H}\mathrm{e}}^{+}$ ions scattered by $129°$ from a Cu surface. Both the primary energy and the angles of incidence and of exit are varied. From our results we conclude the following: along the incoming and outgoing trajectories, neutralization is due to Auger processes and depends on the normal velocity component $v_{\perp }$ only. At higher energies, additional charge exchange is due to collision induced neutralization and reionization, both depending on the total ion energy only. Also in this regime $P^{+}$ depends on $v_{\perp }$, but via a two-valued function of the scattering geometry at fixed energy.