Ionization-excitation of helium by fast protons and antiprotons

The process of simultaneous ionization and excitation of helium by protons and antiprotons p(+/-)+He(1s(2 1)S) --> p(+/-) +e(-) +He+ (2s S-2) is investigated. Total and single differential cross sections for the ionization-excitation process and ionization-excitation probabilities versus the impact parameter are calculated by perturbation theory through the second order in the interaction potential. In the present calculations only the on-energy shell transitions are taken into account. The helium electron wavefunctions for the initial and final states are determined with allowance for the configuration interaction. The normalization of the helium continuous spectrum wavefunctions is examined. The different helium ionization-excitation mechanisms are analysed. The calculations predicted the exceeding of the total cross section for protons over the antiproton one for all collision energies considered. It is shown that the resonant channel coupling causes the ejected electron energy spectrum to have sharp drops at electron energies E-e = E-nl2L - E-2s2S (here, n, l and L are the principal and orbital quantum numbers and the orbital angular momentum for the states of the He+ ion lying above the 2s S-2 one; E-nl2L is the energy of the nl L-2 state of the He+ ion). The present cross sections are compared with those derived from other theoretical approximations.