4.6. Electron Impact Ionization
The availabilility of ion accelerators and storage rings have vastly increased the ability to measure ionization cross sections (e.g. Müller et al. 1985, Müller et al. 1991). But although there have been a much larger number of experiments for ionization than for electron impact excitation, most of the atomic systems, and their ions, have not yet been measured. Much of the experimental work has been carried out at Oak Ridge National Laboratory and the measured cross sections have been compiled in a recent work ``Electron-Impact Ionization of Multicharged Ions at ORNL: 1985-1992'' by D.C.Gregory and M.E. Bannister (1994, ORNL/TM-12729). The experimental data have clearly established the importance of excitation-autoionization (E-A) resonances that can enhance the cross section by orders of magnitude above the background. Theoretical calculations are now carried out for both the direct ionization and the E-A processes. Recent work includes the important iron isonuclear sequence (Pindzola et al. 1987), used by Arnaud and Raymond (1992) in their ionization balance calculations for Iron. Following are general references to available data (see also references in Arnaud and Rothenflug 1985).
1. Critical reviews are by Lennon et al. (1988), Itikawa (1992), Tawara and Kato (1987).
2. Limited data for excited state ionization is given by Golden and Sampson (1980).
3. Hydrogen and Helium ionization is considered by Jones et al. (1991).