Photoionization cross sections of O II, O III, O IV, and O V: benchmarking R-matrix theory and experiments

For crucial tests between theory and experiment, ab initio close coupling calculations are carried out for photoionization of O II, O III, O IV, O V. The relativistic fine structure and resonance effects are studied using the R-matrix and its relativistic variant the Breit Pauli R-matrix (BPRM) approximation. Detailed comparison is made with high resolution experimental measurements carried out in three different set-ups: Advanced Light Source at Berkeley, and synchrotron radiation experiments at University of Aarhus and University of Paris-Sud. The comparisons illustrate physical effects in photoionization such as (i) fine structure, (ii) resolution, and (iii) metastable components. Photoionization cross sections sigma{PI} of the ground and a few low lying excited states of these ions obtained in the experimental spectrum include combined features of these states. Theoretically calculated resonances need to be resolved with extremely fine energy mesh for precise comparison. In addition, prominent resonant features are observed in the measured spectra from transitions allowed with relativistic fine structure, but not in LS coupling. The sigma_{PI} are obtained for ground and metastable (i) 2s^22p^3(^4S^o, ^2D^o, ^2P^o) states of O II, (ii) 2s^22p^2(^3P,^1D,^1S) and 2s2p^3(^5S^o) states of O III, (iii) 2s^22p(^2P^o_J) and 2s2p^2(^4P_J) levels of O IV, and (iv) 2s^2(^1S) and 2s2p(^3P^o,^1P^o) states of O V. It is found that resonances in ground and metastable cross sections can be a diagnostic of experimental beam composition, with potential ap plications to astrophysical and laboratory plasma environments.