Low-energy fine-structure resonances in photoionization of O II

Resonant features in low-energy photoionization cross sections are reported in coupled-channel calculations for O II including relativistic fine structure. The calculations reveal extensive near-threshold resonant structures in the small energy region between the fine structure levels of the ground state 2p(2)(P-3(0,1,2)) of the residual ion O III. Although the resonances have not yet been observed, they are similar to other experimentally observed features. They are expected to significantly enhance the very-low-temperature dielectronic recombination rates, potentially leading to the resolution of an outstanding nebular abundances anomaly. Higher energy partial and total photoionization cross sections of the ground configuration levels 2p(3)(S-4(3/2)degrees, D-2(3/2,5/2)degrees, P-2(1/2,3/2)degrees) are found to be in agreement with experimental measurements on synchrotron-based photon sources [1-3], thereby identifying the excited O III levels present in the ion beams. These are also the first results from a recently developed version of Breit-Pauli R-matrix (BPRM) codes, with inclusion of two-body magnetic interaction terms. The improved relativistic treatment could be important for other astrophysical applications and for more precise benchmarking of experimental measurements.