Electron-impact excitation of Fe XXII: Comparative study of relativistic R-matrix and distorted-wave approaches

An implementation of the Dirac R-matrix theory is presented and applied to the calculation of electron-impact excitation of B-like Fe XXII in the n=2 complex. A detailed comparison between the R-matrix and relativistic distorted-wave (DW) results is given. Contrary to the previous close-coupling studies of this ion, where significantly different background collision strengths are found for several transitions as compared with the DW results, we obtain excellent agreements between the two methods for most transitions, except for the weak transitions to the higher members of the 2p³ configuration, where significant channel-coupling effects are indeed present. We show that the discrepancies found in the previous Dirac R-matrix calculation for dipole transtions at high energies are due to the nonconvergence of partial wave summation, despite the explicit inclusion of partial waves up to l=40. The reason for large differences in the threshold energy region for some transitions between the present and previous Dirac R-matrix results is not clear. We also show that the independent-process, isolated-resonance approximation within the DW framework can describe the near-threshold resonances reasonably well for this ion.