Measurements of Absolute Cross Sections for Electron-Impact Ionization of Doubly Charged Rare Gas Ions: Ne2+, Ar2+, Kr2+, and Xe2+

The technique of crossed electron-ion beams has been employed to measure the absolute cross sections for single ionization of Ne 2+ , Ar 2+ , Kr 2+ and Xe 2+ ions at electron energies ranging from below threshold to 1000 eV. Detailed descriptions of the experimental apparatus and procedures to determine the absolute cross sections are presented, and are discussed check points for reliability and consistency as well as possible contributions of metastable contents in the ion beam. The total systematic error in the cross sections is estimated to be within ±9%. A distinct discrepancy is observed for Ne 2+ between the measured cross section curve and results of semiempirical calculation using the Lotz formula. The cross sections for Ar 2+ , Kr 2+ and Xe 2+ reveal significant contributions from excitation-autoionization processes in addition to direct ionization, especially in the near threshold energy regions.