Excitation of helium by multicharged ions

Abstract

We have performed one-centre two-electron close-coupling calculations on the excitation of helium by bare ions with charge 1-24 and energy 0.1-1.5 MeV amu^-1. Our results are compared with the experiments of Reymann et al. (1988) and Anton et al. (1993), the second Born approximation and the Schwinger approximation. Our study confirms the conclusion reached by Rodriguez and Salin (1992) on the evolution of the excitation process with increasing projectile charge: the transition from a weak perturbation regime which takes place at small impact parameters to a strong interaction regime which favours increasingly large impact parameters. It casts doubts on the possibility of expressing the excitation cross section as a series in the projectile charge Zp associated with the Born expansion and shows that the large Zp behaviour of the excitation probability as a function of impact parameter cannot be described in terms of a saturation concept.