Absolute measurements and calculations of dielectronic recombination with metastable He-like N, F, and Si ions

Measurements and calculations of the rate coefficient 〈vσ〉 for dielectronic recombination associated with He-like ions of nitrogen, fluorine, and silicon are reported for electron energies between 0 and 27 eV. In this energy region, no resonances are associated with excitation of the 1${\mathit{s}}^2$ ground state, however, the metastable states 1s2s${(}^1$S) and 1s2s${(}^3$S) contribute with very large rate coefficients. The population of the 1s2s${(}^3$S) metastable state was determined by an independent x-ray measurement. We performed calculations in the LS-coupling and intermediate-coupling approximations for dielectronic recombination from the initial ${}_{}{}^3{}_{}{}^{}$S and ${}_{}{}^1{}_{}{}^{}$S states. For the ${}_{}{}^3{}_{}{}^{}$S initial state, a comparison on an absolute scale between theory and experiment is presented. We find good agreement with LS-coupling calculations for ${\mathrm{N}}^{5+}$ ${(}^3$S), one exception being a structure observed below the ${}_{}{}^1{}_{}{}^{}$S and ${}_{}{}^3{}_{}{}^{}$P threshold. For ${\mathrm{Si}}^{12+}$ ${(}^3$S), the intermediate-coupling calculation reproduces the data fairly well. In the case of ${\mathrm{F}}^{7+}$ ${(}^3$S), we find discrepancies with both the LS- and intermediate-coupling calculations at low energy and just below the ${}_{}{}^1{}_{}{}^{}$S and ${}_{}{}^3{}_{}{}^{}$P thresholds. The contribution from the initial 1s2s${(}^1$S) state is important only for ${\mathrm{N}}^{5+}$. Here, the experimental data are in excellent agreement with the theory if the ${}_{}{}^1{}_{}{}^{}$S metastable fraction is assumed to be about 1%.