R-matrix calculations for opacities: V. Temperature-density dependence of photoabsorption cross sections and opacity spectra of oxygen ions O VI and O VII
We present R-matrix photoabsorption cross sections for Li-like oxygen O VI and He-like oxygen O VII and examine how plasma broadening modifies them as a function of temperature and density in astrophysical and laboratory high-energy-density (HED) plasma sources. All atomic systems are subject to plasma environment effects, and the propagation of radiation depends on photoabsorption via bound-bound transitions as spectral lines and autoionizing resonances in bound-free photoionization cross sections. We identify and illustrate low and high temperature-density limits for the onset of plasma broadening in O VI and O VII, demonstrating general features and methodology applicable across a broad range of plasma conditions. Calculations are presented along two representative isotherms corresponding to the solar base of convection zone (BCZ) at $T = 1 \times 10^6$ and $2 \times 10^6$ K, and electron densities $N_e = 10^{18}$-$10^{23}$ cm$^{-3}$. Autoionizing resonances progressively dissolve into the continuum with increasing electron density at each isotherm, flattening and merging into the background cross sections at BCZ conditions, at much lower densities than bound-bound line features require. Illustrative examples are given for energy regions containing Rydberg resonance series, including large photoexcitation-of-core (PEC) resonances. Comparisons with previous Opacity Project results reveal that (i) the R-matrix photoionization cross sections cover a much higher energy range where a significantly richer spectrum of autoionizing resonances are present that are not included in OP, and (ii) the corresponding monochromatic opacities show significant quantitative differences. This work is generally applicable to modeling HED plasmas in astrophysics, and to the analysis of transmission spectra in laboratory experiments on inertial confinement fusion (ICF) devices.