Electron-ion recombination and photoionization of Fe XXI

Results for electron-ion recombination and photoionization of (Fe XXI + hv ↔ Fe XXII + e), with emphasis in high-temperature region, are presented from ab initio unified method. The unified method, based on close coupling (CC) approximation and R-matrix method, (i) subsumes both the radiative recombination (RR) and dielectronic recombination (DR), (ii) enables self-consistent sets of photoionization and recombination cross sections from using an identical wavefunction for both the processes, and (iii) provides state-specific recombination rates of a large number of bound states. A large CC wavefunction expansion, which includes the ground and 28 core excitations of n = 2 and 3 complexes and span a wide energy range, has been used. Compared to Δn = 2 - 2, Δn = 2 - 3 core excitations are found to introduce strong resonant structures and enhance the background photoionization cross sections (σPI) in the high-energy region. These features along with prominent photoexcitation-of-core (PEC) resonances at n = 3 core thresholds have increased the unified total recombination rate coefficients α_R(T)) at temperatures T > 10^6 K, region of maximum abundance of the ion in collisional equilibrium, by a factor of 1.6 over previous calculations. State-specific recombination rate coefficients α_R(nLS), which include both the RR and DR, are presented for the first time for 685 bound states with n ≤ 10 and l ≤ 9. The unified total recombination rate with photoelectron energy α_R(E) is presented and the role of low-energy near-threshold fine structure resonances is illustrated. The present results should provide a reasonably complete self-consistent set of recombination rates and photoionization cross sections for astrophysical modelings of high-temperature plasmas from optical to far-ultraviolet wavelength regions.This work was partially supported by the NASA Astronomy and Physics Research Analysis Program. The computational work was carried out on Cray machines at the Ohio Supercomputer Center in Columbus Ohio