Dielectronic recombination (via N = 2→N = 2 core excitations) and radiative recombination of Fe xx: Laboratory measurements and theoretical calculations

We have measured the resonance strengths and energies for dielectronic recombination (DR) of Fe XX forming Fe XIX via N = 2 → N′ = 2 (N = 0) core excitations. We have also calculated the DR resonance strengths and energies using AUTOSTRUCTURE, HULLAC, MCDF, and R-matrix methods, four different state-of-the-art theoretical techniques. On average the theoretical resonance strengths agree to within . 10% with experiment. The AUTOSTRUCTURE, MCDF and R-matrix results are in better agreement with experiment than are the HULLAC results. However, in all cases the 1σ standard deviation for the ratios of the theoretical-to-experimental resonance strengths is & 30% which is significantly larger than the estimated relative experimental uncertainty of . 10%. This suggests that similar errors exist in the calculated level populations and line emission spectrum of the recombined ion. We confirm that theoretical methods based on inverse-photoionization calculations (e.g., undamped R-matrix methods) will severely overestimate the strength of the DR process unless they include the effects of radiation damping. We also find that the coupling between the DR and radiative recombination (RR) channels is small.