L-shell photoionization cross-sections for Fe XIX and Fe XX recombination rate coefficients

High-quality atomic data concerning the interaction of photons with highly ionized ions of iron are an essential requirement in the modelling of X-ray emission from astrophysical plasmas. Consequently a detailed study of the photoionization of the ground-state finestructure levels (2s(2)2p(43)P(2,0,1)) of Fe XIX has been undertaken, and a set of high-resolution cross-sections for photoionization of each of these levels obtained. We have considered both the total and partial cross-section cases, with the latter restricted to those instances where the residual ion is left in one of the energetically lowest 23 J pi (12 LS pi) states of Fe xx. The calculation was performed by applying the Breit-Pauli R-matrix codes using a sophisticated representation of these 23 target-states. This approach allows for the possibility of finestructure splitting in both thresholds and resonances, as well as incorporating any coupling between target-states with different LS symmetries through the inclusion of the spin-orbit operator. A significant amount of such coupling is discovered in the present investigation, and is responsible for a background cross-section whose magnitude is lower than that determined in previous LS-coupling calculations by more than a factor of 2. Extensive resonance structure is also resolved, including shape resonances in the 133 --> 148 Ryd photon energy region. Such structure has not been resolved in any previous investigation and is of extreme importance in the modelling of X-ray emission plasmas. Using the derived photoionization cross-section data, recombination rate coefficients are calculated using the Milne relation, for the case of an electron recombining with Fe XX in the ground state to form Fe XIX existing in each of the fine-structure ground-state levels, These recombination rates are summarized over a temperature range of 50 000 to 10(7) K. To supplement these data further we also present a set of energy levels and oscillator strengths for Fe xx.