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

Abstract

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 ( 3 P 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 ( 4 S o 3/2 , 2 D o 3/2,5/2 , 2 P o 1/2,3/2 ) are found to be in agreement with experimental measurements on synchrotron-based photon sources [1][2][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.

Improved collision strengths and line ratios for forbidden [O iii] far-infrared and optical lines

“…A much larger set of correlation configurations is included for configuration interaction with the spectroscopic terms using the atomic structure code SU-PERSTRUCTURE (Eissner et al 1974, Nahar et al 2003): [1s 2 ]2p 4 , 2s 2 2p3p, 2s 2 2p3d, 2s 2 2p4s, 2s 2 2p4p, 2s2p 2 3s, 2s2p 2 3p, 2s2p 2 3d, 2s 2 3s 2 , 2s 2 3p 2 , 2s 2 3d 2 , 2s 2 4s 2 , 2s 2 4p 2 , 2s 2 3s3p, 2s 2 3s4s, 2s 2 3p3d, 2p 3 3s, 2p 3 3p, 2p 3 3d. We note here that the crucial fine structure separations between the ground state 3 P0,1.2 levels reproduced theoretically agree with experimentally measured values to ∼3% (Nahar et al 2011, see Pradhan and Nahar (2011) for a general description of atomic processes and calculations). The observed energies were substituted for theoretical ones in order to reproduce the threshold energies more accurately.…”

“…( 2) representes the one-body terms of the Breit interaction. In addition, another version of BPRM codes including the two-body terms of the Breit-interaction (Nahar et al 2011; W. Eissner and G. X. Chen, in preparation) has been developed, and is employed in the present work.…”

“…Another recent development in relativistic R ‐matrix codes is the inclusion of the two‐body fine‐structure Breit interaction terms in the Breit–Pauli Hamiltonian (Nahar et al 2011; Eissner & Chen, in preparation). A relativistic calculation of collision strengths can therefore be carried out, including fine structure explicitly and more accurately than in previous works.…”

H Ii Region Metallicity Distribution in the Milky Way Disk

“…García-Rojas et al (2007b) suggest that the problem stems from the presence of temperature fluctuations, whereas Liu et al (2000) argue that it comes from hydrogen deficient clumps in the nebula. Recently, Nahar et al (2010) have suggested that the abundance discrepancy problem is caused by an underestimate of the low-temperature (10 2 −10 4 K) dielectronic recombination rates. Based on theoretical calculations they argue that resonant features in the low-energy photoionization cross section of O ii must be included.…”

Highly excited core resonances in photoionization of Fe xvii: Implications for plasma opacities

“…(3) representes the one-body terms of the Breit interaction. Another version of "full" BPRM codes has been developed including the two-body terms, but those are only of importance in forbidden (E2, M1, M2) transitions where two-electrons correlations may play an important role [25]. In contrast, plasma opacities in toto are determined by strong E1 (dipole allowed and intercombination) considered in this work.…”