Ion Neutralization in Low Energy Ion-Surface Collisions.

A model of ion neutralization in slow (.1 to 3 keV) ion-surface collisions is presented which incorporates both the well-localized (core) and delocalized (band continuum) levels of the solid. The energy levels of the discrete states are found to be broadened by their finite lifetimes and the motion of the ion. This broadening is found to open channels for electronic transitions conventionally prohibited by energy considerations and suggests a certain insensitivity of these transitions to the detailed electronic structure of the band levels of the solid. The formalism allows the complicated sequence of electronic transitions occurring during the collision to be understood. Within certain limitations, the core and band contributions to ion survival may be separated. The charge transfer problem within the (discrete) two-state approximation in the presence of a band of continuum levels has been solved analytically for the case of two avoided crossings. For symmetric, near-resonant collisions the solution reduces in the zero-bandwidth approximation to the standard Landau-Zener fomula with quasi-resonant oscillations. The solution provides a basis for understanding the well-known quasi-resonant electron transfer (QRET) phenomenon observed in Ion Scattering Spectroscopy (ISS). In the off-resonant case, this development represents the first description of the neutralization/reionization events occurring during the period of close, violent collision.