Mechanisms of the double-electron excitation in fast ion-atom collisions: the helium 1s21S &rarr 2s21S excitation

The probabilities and cross sections of the helium 1s21S &rarr 2s21S excitation in collisions of protons, antiprotons and multicharged ions with helium atoms have been calculated with allowance for the configuration interaction at ion energies ranging from 0.1 to 10 MeV/u. The mechanisms for the helium 2a21S autoionizing state excitation are determined, the role of the continuous spectrum states at the helium double-electron excitation is examined. The criterion of the helium atom nucleus effective charge choice at the description of a collision process is established. The comparison of the present results with the available experimental data and with the results received in different theoretical approaches has been made.