Distorted-wave calculations of the electron-impact ionization of highly ionized Na-like ions

We report on distorted-wave calculations of the electron-impact ionization cross sections and rate coefficients for the Na-like ions ${\mathrm{Ti}}^{11+}$, ${\mathrm{Cr}}^{13+}$, ${\mathrm{Fe}}^{15+}$, and ${\mathrm{Ni}}^{17+}$. The cross sections for direct ionization out of the 3s, 2p, and 2s subshells were determined using the configuration-average, distorted-wave approximation. In addition, excitation-autoionization contributions originating from inner-shell excitations of the type 2$s^2$2$p^6$3s→2$s^2$2$p^5$3snl (n=3,4,5) and 2$s^2$2$p^6$3s →2s2$p^6$3snl (n=3,4) were calculated by employing the distorted-wave approximation and configuration-interaction, bound-state wave functions. Near threshold, the total-ionization cross sections in all four ions are enhanced by approximately a factor of 5 through excitation autoionization. These excitation-autoionization contributions are large, even though they have been substantially reduced by radiative decay to the bound states of the initial ion. Furthermore, although electron correlation has a minimal effect on the excitation cross sections, the magnitude of the branching ratios for autoionization is quite sensitive to configuration interaction in the autoionizing levels. This is especially true for the 2p→3d excitations, where configuration interaction between 2$p^5$3s3d and 2$p^5$3$p^2$ can increase the magnitude of the autoionizing rates for levels within the 2$p^5$3s3d configuration by as much as three orders of magnitude. Thus, for example, in the case of the 2p→3d excitation in ${\mathrm{Ni}}^{17+}$, correlation increases the overall branching ratio for autoionization from 0.27 to 0.53.