Double Ionization of Helium by Protons and Electrons at High Velocities

Double ionization of helium at high projectile velocities, upsilon , is considered in terms of two mechanisms. In the shakeoff mechanism the ratio of double- to single-ionization cross sections, sigma super(ii)/ sigma super(i) is independent of upsilon . In the two-step mechanism it is shown that sigma super(ii)/ sigma super(i) similar to ( upsilon super(2)ln upsilon ) super(-1). Combining amplitudes gives a reasonable fit to the observed velocity dependence as well as an explanation of the observed factor-of-2 differences between double ionization of helium by protons and electrons near upsilon approximately equals 10 upsilon sub(Bohr).