The scattering of doubly charged ions from surfaces

Abstract

The scattering of doubly charged ions from clean surfaces provides the opportunity to study electron exchange processes between states of higher binding energy than in the case of the scattering of singly charged ions. Experiments on the scattering of such doubly charged ions have been carried out in the incident energy range 0–10 keV and at 200 keV. At the lower energies, no doubly charged particles are observed to survive the scattering event in a doubly charged state. Substantial neutralisation of the doubly charged incident ion occurs, but the yield of singly charged particles due to a single electron capture by the doubly charged incident ion is greater than the yield of singly charged particles surviving the collision when a beam of initially singly charged particles with the same velocity as the doubly charged particles is incident. Measurements of secondary electron yield, though crude, indicate there is a strong potential emission associated with the scattering of the doubly charged ions. At the higher energy, a beam of doubly charged ions incident on the surface gives rise to neutral, singly charged and doubly charged components in the scattered beam. A low resolution electrostatic analysis system has been used to study the proportions of scattered particles in the various charge states. Preferential neutralisation to the singly charged ion is observed, with the fraction of scattered neutral and singly charged particles being about equal and both fractions being an order of magnitude greater than the fraction of doubly charged particles in the scattered flux. The energy spectra of single and double charged particles show no preferential charge exchange at the surface. The results suggest the ion charge state is determined at the transition through the surface layer.