Electron Impact Single Ionization of Ne²⁺, Ar²⁺, Kr²⁺ and Xe²⁺ Ions

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Absolute cross sections for electron impact single ionization of doubly charged rare gas ions, Ne²⁺, Ar²⁺, Kr²⁺ and Xe²⁺, have been measured in the energy range from below threshold to 1000 eV. The measurements were performed with crossed electron and ion beam technique. The measured cross sections for Ne²⁺ ions are found to be in good agreement with the semiempirical Lotz formula for direct ionization. For Ar²⁺ ions is found a small discrepancy between the observed cross section and the formula at lower energies. For Kr²⁺ and Xe²⁺ ions the measured cross sections exhibit complicated structures from threshold to medium energy and are significantly larger than estimates for the direct ionization. These enhancements and complex structures are believed to be indirect ionization processes, such as excitation-autoionization.