Single and double ionization of He by slow protons and antiprotons

Double and single ionization of He by proton (p) and antiproton (p{}) impact in the energy region below 50 keV is studied theoretically within a molecular picture. As the energy decreases, the ratio of the double- to single-ionization cross section is found to increase for p{} impact and to decrease to p impact in agreement with recent measurements. For p impact, the dominant mechanism for double ionization at the lower energies seems to be sequential ladder climbing by the two electrons through various excited channels. For p{} impact, in contrast, the approaching negative charge distorts the He electron cloud toward the other side of the nucleus and decreases the electron binding energies. These effects enhance the important role of electron-electron, interactions, increasing double ionization.