The double ionization of helium by ion impact

Double ionization is considered within the context of the impact-parameter version of the continuum distorted-wave eikonal initial-state approximation. Starting from the independent electron model, the effective potential describing the ejected electron - residual target interaction is modified so that the single-ionization energy of each electron is half the double-ionization energy of helium. Further, the correlation of the ejected electrons is accounted for by explicitly including the Coulomb density-of-states factor for the electron - electron interaction in the calculation. Total cross sections for proton, alpha particle and impact on helium, over the energy range are presented. It is concluded that the inclusion of electron - electron correlation in the exit channel is crucial if an adequate account of the double-ionization process is to be obtained.