Above-surface neutralization of highly charged ions: The classical over-the-barrier model

The neutralization of highly charged ions during interaction with metallic surfaces is accompanied by the ejection of a large number of secondary electrons. Recent experiments demonstrate two main contributions to this electron ejection process: one from the region below the surface and the second from the above-surface portion of the trajectory. We present a theoretical analysis of the neutralization dynamics above the surface, prior to impact, based on the classical over-the-barrier model. The theory incorporates resonant multielectron capture of conduction electrons, resonant loss into unoccupied states of the conduction band, and intra-atomic Auger deexcitation. The effective barrier potential includes quantum corrections to the classical image potential. The effect of below-barrier (‘‘tunneling’’) transfer is investigated. The solution of a coupled system of rate equations allows the approximate determination of the n-shell populations, the projectile charge state, and the total number of Auger electrons. The calculation describes the transient formation of ‘‘hollow’’ atoms. We find satisfactory agreement with recent data for K Auger yields by Meyer et al. [Phys. Rev. Lett. 67, 723 (1991)].