Second-order Born model for two-electron atomic ionization by fast charged-particle impact

We develop a theoretical model to describe simultaneous ionization excitation and double ionization of a two-electron atomic shell (i.e., He 1s2) by a fast charged-particle impact. The model accounts for the projectile-target interaction to the second order whereas the interaction of the two target electrons ejected into continuum is treated nonperturbatively by the convergent close-coupling (CCC) method. In the second-order term, all intermediate states of the target between two subsequent interactions with the projectile are weighted equally with an average energy denominator (the so-called closure approximation) and only the dipole interaction of the projectile with the target is included. The model is suited to describe two-electron ionization processes with large projectile velocity, small momentum transfer, and small to intermediate energy of the ejected electrons. The model is applied to ionization excitation and double ionization of He by electron and positron impact in kinematics of recent experiments