Differential and total cross sections for antiproton-impact ionization of atomic hydrogen and helium

We develop a method for extracting fully differential cross sections for ionization from an impact-parameter treatment of the collision. The approach uses pseudostates. The method is applied to antiproton-impact ionization of H and He. It is not restricted to antiprotons but can be used with other projectiles. The method automatically includes the interaction between the projectile and the target nucleus. This interaction is shown to be very important in low-energy antiproton ionization. Integrated cross sections for elastic scattering, discrete excitation, ionization, and total scattering are also calculated. For ionization, these are compared with experimental data on H and He. At impact energies greater than 30 keV there is agreement with these data. At lower energies the calculated cross sections for He are in disagreement with the trend of the older experimental data of Andersen and Hvelplund but are in qualitative accord with the recent measurement of Knudsen However, there is no overall quantitative agreement with the new measurements. First-order Born calculations are also presented as a benchmark for checking the pseudostate approximation.