Aspects of charge exchange in ion-atom collisions

Abstract

The straight line semi-classical Impact Parameter method has been modified for use with classical trajectories. Ion-atom collisions have been modelled using wavefunctions expanded in terms of atomic basis states which were centred on either the target or projectile ions. Total and differential charge exchange cross-sections are presented for (^4)He(^++) and (^4)He(^+) collisions within the centre of mass energy range 0.21 kev < E(_em) < 2.5 keV. Results using curved and straight line paths are compared with data from other authors. Significant trajectory effects were found at the lower energies in the range. The curved trajectory results were lower than those from the straight line model and also lower than previous calculations carried out. At higher energies in the range there was good agreement between straight line and curved trajectory models and previous work. Differential cross-sections were found to be sensitive to the trajectories employed, and comparisons have been made with previous work. Total, state specific and differential cross- sections for charge exchange are presented for Be(^++) and H collisions using a five state basis, within the centre of mass energy range 0.111 keV < E(_em) < 0.4444 keV. There was reasonable agreement between the straight line results and previous work. There were significant trajectory effects for all the final charge transfer states. Results are presented for low-energy collisions between positively charged muons and atomic hydrogen. An eight state basis has been used. Direct excitation cross sections for n = 2 atomic states and charge transfer cross sections to Is and n = 2 have been calculated. The effect on the cross sections of using different internuclear potentials has been examined. Trajectory effects were small for charge transfer to Is but were more pronounced in the direct excitation and charge exchange cross- sections to n = 2. These results have been compared to those obtained for curved trajectory H(^+) and H collisions at the same relative velocity, to assess the validity of velocity scaling. It was found that velocity scaling was reliable for charge transfer to Is and for total electron capture cross-sections. However, it was progressively inaccurate for direct excitation and for electron capture into excited states for µ(^+) impact energies of less than 300 eV. These results are discussed and suggestions for further work are made.