# Photoionization and Opacity

**Authors:** Anil Pradhan  
**arXiv:** [2303.03444](https://arxiv.org/abs/2303.03444)  
**Date:** 2023-03-06  
**Summary page:** https://www.summarizepaper.com/en/paper/2303.03444v1/

## Key points

- Photoionization plays a crucial role in determining opacity for radiation transport through material media
- Bound-bound line transitions and bound-free photoionization are primary contributors to atomic opacity in plasma sources
- The R-matrix method is used for state-of-the-art photoionization calculations in the Opacity Project (OP) and Iron Project (IP)
- Plasma broadening of autoionizing resonances due to electron impact, Stark (electric microfields), Doppler (thermal), and core excitations is an unsolved problem
- R-matrix opacity calculations require a huge amount of atomic data and calculations of unprecedented complexity
- In high-energy-density plasmas, photoionization cross sections become 3D energy–temperature–density dependent
- Accuracy depends on the precise energy distribution of differential oscillator strength or photoionization cross section
- The hydrogenic approximation may satisfy the f-sum rule but would be inaccurate when used for complex atoms
- The R-matrix method focuses on differential oscillator df/dE in the bf–continuum based on full delineation of autoionizing resonance profiles
- Excited configurations can be specified straightforwardly in an atomic structure–DW calculation but are more complex and indirect in RM calculations.

## Summary

This article discusses the role of photoionization in determining opacity, which is crucial for understanding radiation transport through material media. In plasma sources, bound-bound line transitions and bound-free photoionization into the continuum are the primary contributors to atomic opacity. The theoretical methodology for state-of-the-art photoionization calculations based on the R-matrix method as employed in the Opacity Project (OP) and Iron Project (IP) is reviewed. The article also addresses the heretofore unsolved problem of plasma broadening of autoionizing resonances due to electron impact, Stark (electric microfields), Doppler (thermal), and core excitations. R-matrix opacity calculations entail a huge amount of atomic data and calculations of unprecedented complexity. In high-energy-density (HED) plasmas, photoionization cross sections become 3D energy–temperature–density dependent owing to considerable attenuation of autoionizing resonance profiles. Hence, differential oscillator strengths and monochromatic opacities are redistributed in energy, significantly affecting Rosseland and Planck mean opacities. The article highlights that while completeness is ensured by the f-sum rule, accuracy depends on the precise energy distribution of differential oscillator strength or photoionization cross section. The hydrogenic approximation may satisfy the f-sum rule but would be inaccurate when used for complex atoms. The R-matrix method (RM) is concerned primarily with differential oscillator df/dE in the bf–continuum based on full delineation of autoionizing resonance profiles. The end result of RMO depends on energy distribution of monochromatic opacity convolved over the Planck function at a given temperature. Compared with OP results, RM Fe xvii monochromatic opacity distribution is quite different and much more smoothed out without sharp variations that stem mainly from treatment of resonances as bb lines even with limited auto ionization broadening included perturbatively in DW opacity models. Finally, this review describes how excited configurations can be specified straightforwardly in an atomic structure–DW calculation but are more complex and indirect in RM calculations. The NP16 BPRM calculations that yield 454 bound levels Fe xvii were complemented with >50,000 highnℓ “top up” levels to compute opacities and RMOs. Atomic and plasma effects such as channel coupling, broadening of auto ionizing resonances, high energy behavior, and oscillator strength sum rule are described.