Double Ionization of Helium by Protons and Electrons at High Velocities

Double ionization of helium at high projectile velocities, v, is considered in terms of two mechanisms. In the shakeoff mechanism the ratio of double- to single-ionization cross sections, σⁱⁱσⁱ, is independent of v. In the two-step mechanism it is shown that σⁱⁱσⁱ~(v²lnv)^-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 v10vBohr.