Relativistic calculations of photoionization cross sections

Relativistic calculations of photoionization cross sections using the Breit-Pauli R-matrix method in the close-coupling approximation and the fully relativistic distorted-wave method are reported. Both methods give fine-structure results; the former produces extensive resonance structures in the low-energy region, and the latter, as a complement to the former, gives accurate results for high energies. Radiation damping of autoionizing resonances, important for He-like and Li-like ions, is included in the R-matrix calculations. In the present work, the Breit-Pauli R-matrix method is employed to calculate photoionization cross sections for photoelectron energies up to 50 Ry in C-like Fe XXI and up to 630 Ry (with radiation damping included) in Li-like Fe XXIV, and the relativistic distorted-wave method is used to calculate high-energy cross sections for He-like through Mg-like ions. The combined relativistic R-matrix and distorted-wave results are expected to provide accurate relativistic photoionization cross sections for highly charged ions throughout the energy range of importance, for practical applications such as photoionization in x-ray astronomy. The Breit-Pauli R-matrix results can also be employed for obtaining photorecombination (subsuming both the direct radiative and the dielectronic recombination) cross sections using the Milne relation. The relativistic distorted-wave results, including inner-shell ionization, for high photon energies may be merged with the low-energy cross sections computed by the Opacity Project using the nonrelativistic R-matrix method.