Ionization of helium by fast, multiply charged ions. Deviations from aq2scaling

Experimental studies of the ionization of helium by fast, light ions have been carried out to investigate deviations from a $q^2$ scaling of the ionization cross section by ions of charge $q$. Systematic studies were performed for equivelocity H, He, Li, B, C, and O ions, carrying up to a maximum of three to four electrons, at reduced energies of 0.64, 1.44, and 2.31 MeV/amu. For ions of low charge, the results show good agreement with the theoretical work of Gillespie and Inokuti, who treat the deviations arising from ionic structure within the framework of the Bethe theory. In contrast, fully stripped ions of sufficiently high nuclear charge show deviations from a $q^2$ scaling, due to the failure of the first Born approximation. Such deviations from the first-order perturbative results were found to be well described by the qualitative approach of Bohr as well as by the three-state, close-coupling approach of Janev and Presnyakov.