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

Abstract

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 it has been done in establishing a universal shape function for single ionization [Rost and Pattard 1997 Phys. Rev. A {\bf 55} R5]. Thereby, it is demonstrated that direct double ionization dominates the cross section for the targets considered.

4 pages, 3 figures

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