ABSOLUTE MEASUREMENTS AND CALCULATIONS OF DIELECTRONIC RECOMBINATION WITH METASTABLE HE-LIKE N, F, AND SI IONS

Measurements and calculations of the rate coefficient [upsilon-sigma] 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 1s2 ground state, however, the metastable states 1s2s(1S) and 1s2s(3S) contribute with very large rate coefficients. The population of the 1s2s(3S) 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 3S and 1S states. For the 3S initial state, a comparison on an absolute scale between theory and experiment is presented. We flnd good agreement with LS-coupling calculations for N5+(3S), one exception being a structure observed below the 1S and 3P threshold. For Si12+(3S), the intermediate-coupling calculation reproduces the data fairly well. In the case of F7+(3S), we find discrepancies with both the LS- and intermediate-coupling calculations at low energy and just below the 1S and 3P thresholds. The contribution from the initial 1s2s(1S) state is important only for N5+. Here, the experimental data are in excellent agreement with the theory if the 1S metastable fraction is assumed to be about 1%.