Photoionization, transition probabilities, and opacities.

Opacity is a measure of radiation transport through matter. Determination of plasma opacities requires radiative data for bound - bound transitions, the oscillator strengths (f-values), and bound - free transitions, the photoionization cross sections (σ[sub PI]), for many levels of the atomic species in all ionization states in the plasma. Hence, large-scale computations are needed to obtain these atomic parameters. Under an international initiative, the Opacity Project (OP), these processes were studied in detail and large-scale ab initio calculations were carried out using the close coupling R-matrix method for the f-values and σ[sub PI] of most astrophysically abundant atoms and ions. Application of these data for stellar opacities solved some outstanding problems in astronomy. We have further extended the OP work to electron -ion recombination (the inverse process), unifying the non-resonant and resonant recombination processes, radiative and di-electronic recombination, heretofore treated independently. A theoretically self-consistent treatment of photoionization and recombination is also thereby enabled. In addition, new relativistic calculations using the Breit-Pauli R-matrix (BPRM) method are being carried out under the Iron Project, a second on-going initiative for high-precision studies of collisional and radiative processes of iron and iron-peak elements. The photoionization and recombination cross sections show excellent agreement with recent experiments. [ABSTRACT FROM AUTHOR]