Relativistic effects in the photoionization of Ne-like iron

We demonstrate that much simpler techniques can reproduce the results of the full Breit-Pauli R-matrix method to include relativistic effects in photoionization calculations. To allow for the fine-structure splitting of channels in the photoionization of ${\mathrm{Fe}}^{16+},$ we have performed three sets of calculations. The first combined an $\mathrm{LS}$ R-matrix calculation with an $\mathrm{LS}-\mathrm{JK}$ frame transformation, using multichannel quantum defect theory. The second used a relativistic random phase approximation based on the Dirac equation. Both methods give resonant photoionization results nearly identical to those from a third calculation using the full Breit-Pauli R-matrix method. An accurate treatment of fine-structure splitting in ${\mathrm{Fe}}^{16+}$ is necessary to realistically include the ${2p}^5{(}^2{P}_{1/2})nl$ resonances which dominate the low-energy photoionization cross section; consequently, in the inverse process of photorecombination, the low-temperature rate coefficient is dominated by the ${2p}^5{(}^2{P}_{1/2})nl$ dielectronic recombination resonances.