Highly excited core resonances in photoionization of Fe xvii: Implications for plasma opacities

Abstract

A comprehensive study of high-accuracy photoionization cross sections is carried out using the relativistic Breit-Pauli $R$-matrix (BPRM) method for ($h\ensuremath{\nu}+\mathrm{Fe}$ xvii $\ensuremath{\rightarrow}$ Fe xviii $+e$). Owing to its importance in high-temperature plasmas, the calculations cover a large energy range, particularly the myriad photoexcitation-of-core (PEC) resonances including the $n=3$ levels not heretofore considered. The calculations employ a close-coupling wave-function expansion of 60 levels of the core ion Fe xviii ranging over a wide energy range of nearly 900 eV between the $n=2$ and $n=3$ levels. Strong-coupling effects due to dipole transition arrays $2{p}^{5}\ensuremath{\rightarrow}2{p}^{4}(3s,3d)$ manifest themselves as large PEC resonances throughout this range and enhance the effective photoionization cross sections orders of magnitude above the background. Comparisons with the erstwhile Opacity Project (OP) and other previous calculations show that the currently available cross sections considerably underestimate the bound-free cross sections. A level-identification scheme is used for spectroscopic designation of the 454 bound fine structure levels of Fe xvii, with $n\ensuremath{\leqslant}10$, $l\ensuremath{\leqslant}9$, and $0\ensuremath{\leqslant}J\ensuremath{\leqslant}8$ of even and odd parities, obtained using the ab initio BPRM method (compared to 181 $\mathit{LS}$ bound states in the OP work). The calculated energies are compared with those available from the National Institute for Standards and Technology database, which lists 63 levels with very good agreement. Level-specific photoionization cross sections are computed for all levels. In addition, partial cross sections for leaving the core ion Fe xvii in the ground state are also obtained. These results should be relevant to modeling of astrophysical and laboratory plasma sources requiring (i) photoionization rates, (ii) extensive nonlocal-thermodynamic-equilibrium models, (iii) total unified electron-ion recombination rates including radiative and dielectronic recombination, and (iv) plasma opacities. We particularly examine PEC and non-PEC resonance strengths and emphasize their expanded role to incorporate inner-shell excitations for improved opacities, as shown by the computed monochromatic opacity of Fe xvii.