Electron-impact-ionization cross sections and rates for highly ionized berylliumlike ions

Electron-impact-ionization cross sections and rates have been calculated for several berylliumlike ions in the $1s^{2}2s^2{}_{}{}^1{}_{}{}^{}S$ ground state and the $1s^{2}2s2p {}_{}{}^3{}_{}{}^{}P$ metastable state using a distorted-wave Born exchange approximation. The ground-state cross section showed only minor deviations from classical scaling along the isoelectronic sequence. The $2s2p {}_{}{}^3{}_{}{}^{}P$ cross section was more than twice that of the ground state at C III, decreasing to a factor of 1.35 greater at Ar XV. Dominant ground-state configuration interaction of the form $2s^2+2\mathrm{}{p}^2$ changed the total cross section by only a few percent. An analytic fit to the distorted-wave results is given, and this allows the calculation of nonrelativistic electron-impact-ionization cross sections and rates for berylliumlike ions. A comparison with available experimental data shows good agreement between measurements and distorted-wave calculations.