Distorted-wave calculations of cross sections for electron-impact ionization of lithiumlike ions

We used distorted-wave and multiconfigurational Dirac-Fock methods to calculate direct and indirect contributions to cross sections for electron-impact ionization of ${\mathrm{C}}^{3+}$, ${\mathrm{N}}^{4+}$, ${\mathrm{O}}^{5+}$, and ${\mathrm{F}}^{6+}$. The features due to excitation autoionization and resonant excitation double autoionization observed in recent cross-beam measurements [G. Hofmann et al., Z. Phys. D 16, 113 (1990)] are well reproduced in our results. Our cross sections are also in good agreement with reported results of nonrelativistic close-coupling calculations [S. S. Tayal and R. J. W. Henry, Phys, Rev. A 42, 1831 (1990); 44, 2955 (1991)] over most of the energy range studied. We examine the effects of radiative decay on the intermediate states, and we show that neglect of certain channels in the close-coupling calculation results in overestimation of the cross sections in the region of the 1s2s2pnl resonances.