SINGLE AND DOUBLE-IONIZATION OF HE BY SLOW PROTONS AND ANTIPROTONS

Double and single ionization of He by proton (p) and antiproton (pBAR) impact in the energy region below 50 keV is studied theoretically wit hin a molecular picture. As the energy decreases, the ratio of the dou ble- to single-ionization cross section is found to increase for pBAR and to decrease for p impact in agreement with recent measurements. Fo r p impact, the dominant mechanism for double ionization at the lower energies seems to be sequential ladder climbing by the two electrons t hrough various excited channels. For pBAR impact, in contrast, the app roaching negative charge distorts the He electron cloud toward the oth er side of the nucleus and decreases the electron binding energies. Th ese effects enhance the important role of electron-electron interactio ns, increasing double ionization.