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 (ΔN = 0) core excitations. We have also calculated the DR resonance strengths and energies using the AUTOSTRUCTURE, Hebrew University Lawrence Livermore Atomic Code (HULLAC), Multiconfiguration Dirac-Fock (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 1 σ standard deviation for the ratios of thetheoretical-to-experimental resonance strengths is ≳30%, which is significantly larger than the estimatedrelative experimental uncertainty of ≲10%. This suggests that similar errors exist in the calculated levelpopulations 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 thecoupling between the DR and radiative recombination (RR) channels is small. Below 2 eV the theoretical resonance energies can be up to ≈30% larger than experiment. This is larger than the estimated uncertainty in the experimental energy scale (≲0.5% below ≈25 eV and ≲0.2% forhigher energies) and is attributed to uncertainties in the calculations. These discrepancies makes DR of Fe XX an excellent case for testing atomic structure calculations of ions with partially filled shells. Above 2 eV, agreement between the theoretical and measured energies improves dramatically with the AUTOSTRUCTURE and MCDF results falling within 2% of experiment, the R-matrix results within 3%, and HULLAC within 5%. Agreement for all four calculations improves as the resonance energy increases. We have used our experimental and theoretical results to produce Maxwellian-averaged rate coefficients for ΔN = 0 DR of Fe XX. For kBTe ≳ 1 eV, which includes the predicted formation temperatures for Fe XX in an optically thin, low-density photoionized plasma with cosmic abundances, the experimental and theoretical results agree to better than ≈15%. This is within the total estimated experimental uncertainty limits of ≲20%. Agreement below ≈1 eV is difficult to quantify due to current theoretical and experimental limitations. Agreement with previously published LS-coupling rate coefficients is poor, particularly for kBTe ≲ 80 eV. This is attributed to errors in the resonance energies of these calculations as well as the omission of DR via 2p1/2 → 2p3/2 core excitations. We have also used our R-matrix results, topped off using AUTOSTRUCTURE for RR into J ≥ 25 levels, to calculate the rate coefficient for RR of Fe XX. Our RR results are in good agreement with previously published calculations. We find that for temperatures as low as kBTe ≈ 10-3 eV, DR still dominates over RR for this system.