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 sui table dimensionless units. The shape can be represented with a simple analy tical function, following the same principles as those which have been util ized during the establishment of a universal shape function for single ioni zation (Rost and Pattard 1997 Phys. Rev. A 55 R5). Thereby, it is demonstra ted that direct double ionization dominates the cross section for the targe ts considered.