Double Auger processes in the electron-impact ionization of lithiumlike ions

Contributions to the electron-impact ionization of Li-like ions due to resonant recombination followed by double Auger emission are calculated using first- and second-order perturbation theory. The double Auger rate is found to be quite sensitive to the choice of both continuum phase and excited orbital basis set. In particular we find that resonant recombination to the 1s2$s^2$2p ${\mathrm{}}^3$P term of ${\mathrm{O}}^{4+}$, followed by double Auger emission to the 1$s^2$ ${}_{}{}^1{}_{}{}^{}S$ term of ${\mathrm{O}}^{6+}$, has a cross section ranging from 6.4×${10}^{\mathrm{-}22}$ to 4.9×${10}^{\mathrm{-}21}$ ${\mathrm{cm}}^2$ depending on how the double Auger rate is calculated. The maximum represents 0.65% of the total ionization cross section for ${\mathrm{O}}^{5+}$ at the resonance energy of 436.3 eV. Thus a noise level of 1.0%, which is probably a lower limit of current crossed-beam experiments, will mask the contribution in ${\mathrm{O}}^{5+}$. Similar results are found for other Li-like ions in the first-row Periodic Table elements.