Knowledge Bank

Relativistic and channel coupling effects in photoionization and unified electronic recombination of Fe XVII are demonstrated with an extensive 60-level close-coupling calculation using the Breit-Pauli R-matrix method. A multiconfiguration eigenfunction expansion up to the n = 3 levels of the core ion Fe XVIII is employed with five spectroscopic configurations 2s^2 2p^5, 2s2p^6, 2s^2 2p^4 3s, 3p, 3d, and a number of correlation configurations. The unified e + ion recombination calculations for e + Fe XVIII → Fe XVII include both the nonresonant and resonant recombination ("radiative" and "dielectronic recombination"—RR and DR). Photoionization and e + ion recombination calculations are carried out for the total and the level-specific cross sections, including the ground and several hundred excited bound levels of Fe XVII (up to fine-structure levels with n = 10). The low-energy and the high-energy cross sections are compared from (i) a three-level calculation including only the 2s^2 p^5 (^2 Pº_1/2,3/2) and 2s2p^6 (^2 S_1/2) levels of Fe XVIII, and (ii) the first 60-level calculation with Δn > 0 coupled channels. Strong channel coupling effects are demonstrated throughout the energy ranges considered, in particular via giant photoexcitation-of-core (PEC) resonances due to L-M shell dipole transition arrays 2p^5 → 2p^4 3s,3d in Fe XIII that enhance effective cross sections by orders of magnitude. Comparison is made with previous theoretical and experimental works on photoionization and recombination that considered the relatively small low-energy region (i), and the weaker Δn = 0 couplings. While the simpler three-level results describe the near-threshold photoionization and recombination, they are inadequate for practical applications that also require the higher-energy cross sections for modeling ionization balance of Fe XVII in laboratory and astrophysical plasmas. The present 60-level results should provide reasonably complete and accurate datasets for both photoionization and e + ion recombination of Fe XVII.