Universal shape function for the double-ionization cross section of negative ions by electron impact

It is shown that recently measured cross sections for double ionization of negative ions (H - , O - and C - ) possess a universal shape when plotted in suitable dimensionless units. The shape can be represented with a simple analytical function, following the same principles as those which have been utilized during the establishment of a universal shape function for single ionization (Rost and Pattard 1997 Phys. Rev. A 55 R5). Thereby, it is demonstrated that direct double ionization dominates the cross section for the targets considered.