Electron-Iron Recombination Rate Coefficients and Photoionization Cross Section for S XIV and SXV for X-ray and UV modeling

Abstract

The inverse processes of photoionization and electron-ion recombination of (hv + S XIV 1:::; S XV + e) and (hv + S XV =r SXVI+ e) are studied in detail using ab initio unified method that provides self-consistent sets ofresults for these processes. Results are presented for large number of fine structure levels where n S 10 and 0 ~ l ~ 9; 98 levels for Li-Iike S XIV with 1/2 S J ~. 17/2 and 188 levels for He-like S XV with 0 ::;J ~ 10. Photoionization cross section, O'PI, of the levels of both S XIV and S XV decay smoothly in the lower region. However, narrow and high peak autoionizing Rydberg series of resonances belonging to various excited core levels appear in the high energy region and enhance the background cross section of the excited levels. The resonance series of n=2 core levels dominate while they become weaker with higher n. The prominent feature is the enhancement of the background cross section at n=2 core thresholds due toK-shell ionization leaving the ion in excited 2p states. 0' PI also show wide PEe (photo-excitation-of-core) resonances at the photon energies that equal to the core excitation energies. Level-specific photoionization cross sections, O'PJ(nSLJ), and recombination rate coefficients, QRc(nSLJ), are obtained for the first time for theseions. Currently available results correspond to photoionization for L8 terms with missing features and to only total recombination rate coefficients. Present QRc(nSLJ) incorporates both the radiative recombination. (RR) and dielectronic recombination (DR), and show a 'bump' or 'shoulder' in the high temperature region due to DR dominance. The total unified recombination rate coefficients show good agreement with the availableRR and DR rates. Recombination rates over photoelectron energy are presented for astrophysical and laboratory plasma applications. Total recombination rates for H-like S XVI are given for completeness. The results should be accurate within 10-15% based on the unified method that includes important atomic effects such as radiation damping, channel couplings, interference of DR and RR, and relativistic fine structure effects. The comprehensive datasets are applicable for various models such as for ionization balance and recombination-cascade for UV'and X-ray lines.