Experimentally Derived Dielectronic Recombination Rate Coefficients for Heliumlike C V and Hydrogenic O VIII

Abstract

Using published measurements of dielectronic recombination (DR) resonance strengths and energies for C V to C IV and O VIII to O VII, we have calculated the DR rate coefficient for these ions. Our derived rates are in good agreement with multiconfiguration, intermediate-coupling and multiconfiguration, fully-relativistic calculations as well as with most LS coupling calculations. Our results are not in agreement with the recommended DR rates commonly used for modeling cosmic plasmas. We have used theoretical radiative recombination (RR) rates in conjunction with our derived DR rates to produce a total recombination rate for comparison with unified RR+DR calculations in LS coupling. Our results are not in agreement with undamped, unified calculations for C V but are in reasonable agreement with damped, unified calculations for O VIII. For C V, the Burgess general formula (GF) yields a rate which is in very poor agreement with our derived rate. The Burgess & Tworkowski modification of the GF yields a rate which is also in poor agreement. The Merts et al. modification of the GF yields a rate which is in fair agreement. For O VIII the GF yields a rate which is in fair agreement with our derived rate. The Burgess & Tworkowski modification of the GF yields a rate which is in good agreement. And the Merts et al. modification yields a rate which is in very poor agreement. These results suggest that for DN=1 DR it is not possible to know a priori which formula will yield a rate closer to the true DR rate. We describe the technique used to obtain DR rate coefficients from laboratory measurements of DR resonance strengths and energies. For use in plasma modeling, we also present easy-to-use fitting formulae for the experimentally derived DR rates.