Relativistic fine structure oscillator strengths for Li-like ions: C IV - Si XII, S XIV, Ar XVI, Ca XVIII, Ti XX, Cr XXII, and Ni XXVI

Ab initio calculations including relativistic effects employing the Breit-Pauli R-matrix (BPRM) method are reported for fine structure energy levels and oscillator strengths upto n = 10 and 0.leq. l .leq.9 for 15 Li-like ions: C IV, N V, O VI, F VII, Ne VIII, Na IX, Mg X, Al XI, Si XII, S XIV, Ar XVI, Ca XIII, Ti XX, Cr XXII, and Ni XXVI. About one hundred bound fine structure energy levels of total angular momenta, 1/2 .leq. J .leq. 17/2 of even and odd parities, total orbital angular momentum, 0 .leq L .leq. 9 and spin multiplicity (2S+1) = 2, 4 are considered for each ion. The levels provide almost 900 dipole allowed and intercombination bound-bound transitions. The BPRM method enables consideration of large set of transitions with uniform accuracy compared to the best available theoretical methods. The CC eigenfunction expansion for each ion includes the lowest 17 fine structure energy levels of the core configurations 1s^2, 1s2s, 1s2p, 1s3s, 1s3p, and 1s3d. The calculated energies of the ions agree with the measured values to within 1% for most levels. The transition probabilities show good agreement with the best available calculated values. The results provide the largest sets of energy levels and transition rates for the ions and are expected to be useful in the analysis of X-ray and EUV spectra from astrophysical sources.Comment: 16 pgs., to appear in Astronomy and Astrophysic

Recombination Rate Coefficients for KLL Di-electronic Satellite Lines of Fe XXV and Ni XXVII

The unified method for total electron-ion recombination is extended to study the dielectronic satellite (DES) lines. These lines, formed from radiative decay of autoionizing states, are highly sensitive temperature diagnostics of astrophysical and laboratory plasma sources. The computation of the unified recombination rates is based on the relativistic Breit-Pauli R-matrix method and close coupling approximation. Extending the theoretical formulation developed earlier we present recombination rate coefficients for the 22 satellite lines of KLL complexes of helium-like Fe XXV and Ni XXVII. The isolated resonance approximation, commonly used throughout plasma modeling, treats these resonances essentially as bound features except for dielectronic capture into, and autoionization out of, these levels. A line profile or cross section shape is often assumed. On the other hand, by including the coupling between the autoionizing and continuum channels, the unified method gives the intrinsic spectrum of DES lines which includes not only the energies and strengths, but also the natural line or cross section shapes. A formulation is presented to derive autoionization rates from unified resonance strengths and enable correspondence with the isolated resonance approximation. While the rates compare very well with existing rates for the strong lines to <20%, the differences for weaker DES lines are larger. We also illustrate the application of the present results to the analysis of K ALPHA complexes observed in high-temperature X-ray emission spectra of Fe XXV and Ni XXVII. There are considerable differences with previous results in the total KLL intensity for Fe XXV at temperatures below the temperature of maximum abundance in coronal equilibrium. (Abbreviated Abstract)Comment: 21 pages, 5 figures, to appear in Physica Script

Electron impact ionization of metastable 2P-state hydrogen atoms in the coplanar geometry

AbstractTriple differential cross sections (TDCS) for the ionization of metastable 2P-state hydrogen atoms by electrons are calculated for various kinematic conditions in the asymmetric coplanar geometry. In this calculation, the final state is described by a multiple-scattering theory for ionization of hydrogen atoms by electrons. Results show qualitative agreement with the available experimental data and those of other theoretical computational results for ionization of hydrogen atoms from ground state, and our first Born results. There is no available other theoretical results and experimental data for ionization of hydrogen atoms from the 2P state. The present study offers a wide scope for the experimental study for ionization of hydrogen atoms from the metastable 2P state

Electron-Ion Recombination Rate Coefficients and Photoionization Cross Sections for Astrophysically Abundant Elements. IX. Ni XXVI and Ni XXVII for UV and X-ray modeling

The inverse processes of photoionization and electron-ion recombination of (hnu + Ni XXVI --> Ni XXVII + e and (hnu + Ni XXVII --> Ni XXVIII + e) are studied using the unified method for the total recombination. The method subsumes both the radiative and di-electronic recombination processes and enables self-consistent sets of results for photoionization and electron-ion recombination by using the same wavefunction expansion. Photoionization cross sections (sigma_PI), recombination cross sections (sigma_{RC}), recombination collision strengths (Omega_{RC}), and recombination rate coefficients (alpha_{RC}) are obtained for ionization balance and spectral analysis of UV and X-ray lines. Level-specific photoionization cross sections and recombination rates are presented to enable accurate computation of recombination-cascade matrices for all fine structure levels n(SLJ) up to n <= 10: 98 bound fine structure levels of Ni XXVI with 0 <= l <= 9, 0 <= L <= 11, 1/2 <= J <= 17/2, and 198 levels of Ni XXVII with 0 <= l <= 9, 0 <= L <= 14, 0 <= J <= 10. Total alpha_{RC} for Ni XXVI and Ni XXVII are compared with the existing values with very good agreement. Total recombination rate coefficients for the hydrogen-like recombined ion, Ni XXVIII, are also presented. The calculations are carried out in relativistic Breit-Pauli R-matrix (BPRM) approximation with inclusion of radiation damping of resonances. With consideration of all details of the processes, the results, which include the level specific sigma_{PI} and alpha_{R} calculated for the first time, should be the most accurate for these ions.Comment: 27 pages, 10 figures, to appear in Astrophys. J. Supplement

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.Comment: 27 pages, 7 figs., submitted to Phys. Rev. A., text with high resolution figures at http://www.astronomy.ohio-state.edu/~pradhan/Oions.p

K-shell dielectronic resonances in photoabsorption: differential oscillator strengths for Li-like C IV, O VI, and Fe XXIV

Recently X-ray photoabsorption in KLL resonances of O VI was predicted [Pradhan, Astrophys.J. Lett. 545, L165 (2000)], and detected by the Chandra X-ray Observatory [Lee et al, Astrophys. J. {\it Lett.}, submitted]. The required resonance oscillator strengths f_r, are evaluated in terms of the differential oscillator strength df/de that relates bound and continuum absorption. We present the f_r values from radiatively damped and undamped photoionization cross sections for Li-like C,O, and Fe calculated using relativistic close coupling Breit-Pauli R-matrix method. The KLL resonances of interest here are: 1s2p (^3P^o) 2s [^4P^o_{1/2,3/2}, ^2P^o_{1/2,3/2}] and 1s2p (^1P^o) 2s [^2P^o_{1/2,3/2}]. The KLL photoabsorption resonances in Fe XXIV are fully resolved up to natural autoionization profiles for the first time. It is demonstrated that the undamped f_r independently yield the resonance radiative decay rates, and thereby provide a precise check on the resolution of photoionization calculations in general. The predicted photoabsorption features should be detectable by the X-ray space observatories and enable column densities in highly ionized astrophysical plasmas to be determined from the calculated f_r. The dielectronic satellites may appear as redward broadening of resonances lines in emission and absorption.Comment: 9 pages, 2 figurs, Phys. Rev. A, Rapid Communication (submitted

Electron-Ion Recombination Rate Coefficients and Photoionization Cross Sections for Astrophysically Abundant Elements. V. Relativistic calculations for Fe XXIV and Fe XXV for X-ray modeling

Photoionization and recombination cross sections and rate coefficients are calculated for Li-like Fe XXIV and He-like Fe XXV using the Breit-Pauli R-matrix (BPRM) method. A complete set of total and level-specific parameters is obtained to enable X-ray photoionization and spectral modeling. The ab initio calculations for the unified (e + ion) recombination rate coefficients include both the non-resonant and the resonant recombination (radiative and di-electronic recombination, RR and DR, respectively) for (e + Fe XXV) -> Fe XXIV and (e + Fe XXVI) -> Fe XXV. The level specific rates are computed for all fine structure levels up to n = 10, enabling accurate computation of recombination-cascade matrices and effective rates for the X-ray lines. The total recombination rate coefficients for both Fe XXIV and Fe XXV differ considerably, by several factors, from the sum of RR and DR rates currently used to compute ionization fractions in astrophysical models. As the photoionization/recombination calculations are carried out using an identical eigenfunction expansion, the cross sections for both processes are theoretically self-consistent; the overall uncertainty is estimated to be about 10-20%. All data for Fe XXIV and Fe XXV (and also for H-like Fe XXVI, included for completeness) are available electronically.Comment: 31 pages, 10fug

X-ray absorption via K-alpha resonance complexes in oxygen ions

The K-alpha resonance complexes in oxygen ions O I - O VI are theoretically computed and resonance oscillator strengths and wavelengths are presented. The highly resolved photoionization cross sections, with relativistic fine structure, are computed in the coupled channel approximation using the Breit-Pauli R-matrix method. A number of strong K-alpha resonances are found to be appreciable, with resonance oscillator strengths f_r > 0.1. The K-alpha resonance wavelengths of O I-O VI lie in a relatively narrow wavelength range 22 - 23.5 A, and the X-ray opacity in this region should therefore be significantly affected by K --> L transitions in oxygen. The results should be useful in the interpretation of soft X-ray spectra observed from Chandra and XMM-Newton.Comment: Monthly Notices of Roy. Astro. Soc. (in press), 10 pgs. 1 figur

R-Matrix calculations for opacities: I. Methodology and computations

An extended version of the R-matrix methodology is presented for calculation of radiative parameters for improved plasma opacities. Contrast and comparisons with existing methods primarily relying on the Distorted Wave (DW) approximation are discussed to verify accuracy and resolve outstanding issues, particularly with reference to the Opacity Project (OP). Among the improvements incorporated are: (i) large-scale Breit-Pauli R-matrix (BPRM) calculations for complex atomic systems including fine structure, (ii) convergent close coupling wave function expansions for the (e+ion) system to compute oscillator strengths and photoionization cross sections, (iii) open and closed shell iron ions of interest in astrophysics and experiments, (iv) a treatment for plasma broadening of autoionizing resonances as function of energy-temperature-density dependent cross sections, (v) a "top-up" procedure to compare convergence with R-matrix calculations for highly excited levels, and (vi) spectroscopic identification of resonances and bound \eion levels. The present R-matrix monochromatic opacity spectra are fundamentally different from OP and lead to enhanced Rosseland and Planck mean opacities. An outline of the work reported in other papers in this series and those in progress is presented. Based on the present re-examination of the OP work, it is evident that opacities of heavy elements require revisions in high temperature-density plasma sources.Comment: 16 pages, 2 figure

Electron-Ion Recombination Rate Coefficients and Photoionization Cross Sections for Astrophysically Abundant Elements; 4, Relativistic calculations for C IV and C V for UV and X-ray modeling

The first complete set of unified cross sections and rate coefficients are calculated for photoionization and recombination of He- and Li-like ions using the relativistic Breit-Pauli R-matrix method. We present total, unified (e + ion) recombination rate coefficients for (e + C VI ---> C V) and (e + C V \longrightarrow C IV) including fine structure. Level-specific recombination rate coefficients up to the n = 10 levels are also obtained for the first time; these differ considerably from the approximate rates currently available. Applications to recombination-cascade coefficients in X-ray spectral models of K-alpha emission from the important He-like ions is pointed out. The overall uncertainty in the total recombination rates should not exceed 10-20%. Ionization fractions for Carbon are recomputed in the coronal approximation using the new rates. The present (e + ion) recombination rate coefficients are compared with several sets of available data, including previous LS coupling results, and `experimentally derived' rate coefficients. The role of relativistic fine structure, resolution of resonances, radiation damping, and interference effects is discussed. Two general features of recombination rates are noted: (i) the non-resonant (radiative recombination) peak as E,T ---> 0, and the (ii) the high-T resonant (di-electronic recombination) peak