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

Abstract

For crucial tests between theory and experiment, ab initio close-coupling calculations are carried out for photoionization of several oxygen ions: O II, O III, O IV, and 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) approximations. Extremely detailed comparison is made with high-resolution experimental measurements carried out in three different setups: Advanced Light Source at Berkeley, synchrotron radiation experiment with a new modulator at University of Aarhüs, and synchrotron radiation experiment at 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 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. The general features of the measured PI were predicted in the earlier calculations in LS coupling. However, due to higher-resolution achievement in the recent experiments, the theoretically calculated resonances need to be resolved with much finer energy mesh for precise comparison. In addition, prominent resonant features are observed in the measured spectra that can form from transitions allowed with relativistic fine structure, but not in LS coupling. Present results include calculations from both LS coupling and the relativistic BPRM method to decipher the features in photoionization spectra. PI are obtained for ground and metastable (i) Most of the computed theoretical resonance structures are seen experimentally, although there are some discrepancies. One of the main conclusions of the present study is that resonances in photoionization cross sections of the ground and metastable states can be a diagnostic of experimental beam composition, with potential applications to astrophysical and laboratory plasma environments.

Resolution and accuracy of resonances in R-matrix cross sections

“…Although this does not guarantee complete resolution and the OP cross sections may still be under-resolved for certain applications such as monochromatic spectral modelling, they are considerably better than those with constant energy mesh. The new results reported in this work are obtained as in the recent calculations for a number of oxygen ions, O II-O V, using a more extensive eigenfunction expansion for the target ion than in the OP work; they are in good detailed agreement with recent experimental measurements (Nahar 2004). Extensive resonance structures are delineated, especially in the near-threshold region.…”

“…Nevertheless, R-matrix calculations for photoionization cross sections have been experimentally verified, some computed in LS coupling prior to experiments. For example, we note the extremely detailed comparison between R-matrix cross sections and recent experiments on synchrotron based light sources reported by the Reno/Berkeley group for O II (Covington et al 2001), the Aarhus group for C II (Kjeldsen et al 1999, Nahar 2002) and the Paris group for O ions (Champeaux et al 2003, Nahar 2004). Near-threshold resonances need to be carefully resolved, often with relativistic fine structure.…”

“…The unified scheme to obtain total recombination rates, including RR and DR, entails: (A) highly resolved calculations of photoionization cross sections of a large number of bound levels, typically several hundred levels with effective quantum number ν ≤ 10 and (B) DR calculations using an adaptation of the theory by Bell and Seaton (1985), (Nahar and Pradhan 1994a) for 10 < ν ≤ ∞. In addition, for any given ion self-consistency is assured in an ab initio manner between total photoionization and recombination since the same wavefunction expansion is employed for both processes (see Nahar (2004), and references therein). Another advantage of the method is that level-specific cross sections and rates for all levels with ν ≤ 10 are also obtained.…”

Absolute cross sections for L-shell photoionization of the ions N$\mathsf{^{2+}}$, N$\mathsf{^{3+}}$, O$\mathsf{^{3+}}$, O$\mathsf{^{4+}}$, F$\mathsf{^{3+}}$, F$\mathsf{^{4+}}$ and Ne$\mathsf{^{4+}}$

“…The R-matrix results (middle panels) are from Fernley et al (1999, from the Opacity Project) and Nahar & Pradhan (1997), with additional data for O 3+ in Fig. 5 by Nahar (1998Nahar ( , 2004 The experimental spectra show different resonances superimposed on decreasing continua and exhibit marked differences along the series N 2+ -F 4+ . A continuous decrease of the continuum cross section is observed going through the series while the ionization threshold increases.…”

“…Employing the same method, Nahar & Pradhan (1997) calculated the cross sections of the ions of C, N, and O (including N 2+ and O 3+ ) as part of the Iron Project, and later Nahar (1998) reported details of the calculations for oxygen and its ions. All the above investigations were non-relativistic, but after that experiments had demonstrated that such an approach was insufficient to describe the spectrum of C + (Kjeldsen et al 1999a), R-matrix calculations including relativistic effects have been carried out for oxygen ions (Nahar 2004; see also Champeaux et al 2003).…”