Dielectronic recombination from the ground and excited states ofC4+andO6+

We have calculated dielectronic recombination (DR) cross sections from the ground state (1 ${}_{}{}^1{}_{}{}^{}S$) and excited states (2 ${}_{}{}^3{}_{}{}^{}S$, $2^1$S, and 2 ${}_{}{}^3{}_{}{}^{}P$) of the He-like ions ${\mathrm{C}}^{4+}$ and ${\mathrm{O}}^{6+}$ in the LS-coupling and intermediate-coupling approximations, using the program a u t o s t r u c t u r e. We find the effects of configuration interaction, intermediate coupling, and external electric fields to be small. We compare our results with the recent high-resolution measurements of DR from the excited states of these ions by Andersen et al. [Phys. Rev. Lett. 62, 2656 (1989) and Phys. Rev. A 41, 1293 (1990)] by convoluting our cross sections with their electron velocity distribution. For the case of ${\mathrm{C}}^{4+}$, we obtain excellent agreement when we choose the 2 ${}_{}{}^3{}_{}{}^{}S$ to 2 ${}_{}{}^1{}_{}{}^{}S$ population ratio to be 18. For the case of ${\mathrm{O}}^{6+}$, we obtain good agreement with experiment with a 2 ${}_{}{}^3{}_{}{}^{}S$ to $2^1$S population ratio of 70, except in the 6- to 8-eV energy region, where a model calculation indicates that coupling between resonances via the continuum may be important. By comparing experiment with theory, we have also estimated the metastable fraction of the 2 ${}_{}{}^3{}_{}{}^{}S$ term to be 70% in ${\mathrm{C}}^{4+}$ and 20% in ${\mathrm{O}}^{6+}$. The reason for this large difference is not understood.