Abstract
Ionisation of atomic systems by fast ions is considered. A second-order quantum theory is derived which correctly accounts for all distortions of the initial- and final-state wavefunctions arising from Coulomb interactions between three charged particles. The present transition amplitude is the rigorous first-order (or lowest order) approximation to the exact scattering amplitude for ionisation treated by the Faddeev equations in the distorted wave formalism. The problem is briefly studied by the impact parameter method as well, and its equivalence with the full quantal treatment is proved (for projectiles mainly scattered forward) in the limit of very large values of the reduced mass. Ionisation of atomic hydrogen by proton impact is examined in considerable detail at incident energies between 25 and 200 keV. Comprehensive comparisons of available experimental data with singly differential as well as total cross sections for electron ejection are made and the validity of the present method is assessed.