State-selective dielectronic-recombination measurements for He-like oxygen ions in an electron cooler

We report on high-resolution, state-selective dielectronic-recombination measurements for metastable, He-like ${\mathrm{O}}^{6+}$ ions. The measurements were performed with a merged-beams configuration in an electron cooler. The energy resolution was found to be as low as 0.135 eV. Dielectronic recombination associated with excitations of the 1s2s${(}^1$S) and 1s2s${(}^3$S) cores are reported. The data demonstrate strong effects of configuration interaction between different intermediate states with almost identical energy.