Dielectronic Recombination of Fe Xxi and Fe Xxii via N = 2!n 0 = 2 Core Excitations

We have measured dielectronic recombination (DR) resonance strengths and energies for carbon-like Fe xxi forming Fe xx and for boron-like Fe xxii forming Fe xxi via N 1⁄4 2 ! N 0 1⁄4 2 core excitations. All measurements were carried out using the heavy-ion Test Storage Ring at the Max-Planck-Institute for Nuclear Physics in Heidelberg, Germany. We have also calculated these resonance strengths and energies using three independent, state-of-the-art perturbative techniques: a multiconfiguration Breit-Pauli (MCBP) method using the code AUTOSTRUCTURE, a multiconfiguration Dirac-Fock (MCDF) method, and a relativistic configuration interaction method using the Flexible Atomic Code (FAC). Overall, reasonable agreement is found between our experimental results and our theoretical calculations. Themost notable discrepancies tend to occur for relative collision energies d3 eV. We have used our measured 2 ! 2 results to produce Maxwellianaveraged rate coefficients for Fe xxi and Fe xxii. Our experimentally derived rate coefficients are estimated to be accurate to better than 20% both for Fe xxi at kBTe > 0:5 eV and for Fe xxii at kBTe > 0:001 eV. For these results, we provide fits that are accurate to better than 0.5% for Fe xxi at 0:001 eV kBTe 10; 000 eV and for Fe xxii at 0:02 eVdkBTe 10; 000 eV. Our fitted rate coefficients are suitable for ionization balance calculations involving Fe xxi and Fe xxii in photoionized plasmas. Previous published Burgess formula and LS-coupling calculations are in poor agreement with our experimentally derived rate coefficients. None of these published calculations reliably reproduce the magnitude or temperature dependence of our experimental results. Our previously published Fe xxiMCDF results are in good agreement with our experimental results for kBTee0:07 eV. For both ions in this temperature range our new MCBP, MCDF, and FAC results are in excellent agreement with our experimentally derived rate coefficient. Subject headings: atomic data — atomic processes — methods: laboratory