Double ionization of helium by proton impact: A generalized-Sturmian approach

We present ab initio calculations for the double ionization of helium by fast proton impact, using the generalized-Sturmian-functions methodology and within a perturbative treatment of the projectile-target interaction. The cross-section information is extracted from the asymptotic behavior of the numerical three-body function that describes the emission process. Our goal is to provide benchmark first-order Born fully differential cross sections with which one may investigate the suitability of transition matrices calculated using approximate analytic-type solutions for the double continuum (the choice of effective charges or effective momenta to partially account for the internal target interactions being, to some extent, arbitrary). We also provide fully differential cross sections for the low-ejection-energy regime, which is beyond the suitable range of such perturbative methods. We find, however, that the effective momentum approach allows one to get at least a rough characterization of the most dominant physical process involved. We also compare our calculations with the only available relative experimental set, showing an agreement in shape that can be well understood within the given momentum transfer regime.