Collisions between positive ions and free electrons are important atomic processes in high temperature plasmas. These collisions influence the distribution of ionic charge states and atomic level populations in the plasmas. Accurate knowledge of cross sections for electron-impact ionization is critical for interpreting the spectra observed in both laboratory and astrophysical plasmas.

Earlier studies of electron-impact ionization indicated that indirect processes can significantly increase the ionization cross sections for ions with low atomic number, and It was generally assumed that these indirect processes would become negligible for heavier ions. However, theoretical studies at Lawrence Livermore National Laboratory led to predictions that the effects of indirect processes could persist for heavy ions. These predictions were verified when techniques for measuring ionization cross sections for heavier ions were eventually developed.

In this presentation the simple atomic physics underlying our studies of indirect processes will be described, and the significance of these findings will be discussed. Some experiments at Lawrence Livermore National Laboratory where these results have applications will also be shown.