Oscillator strengths for dipole-allowed fine-structure transitions in Si II

An extensive dataset of oscillator strengths, line strengths, and Einstein A-coefficients has been calculated for a large number of dipole-allowed (ΔS=0) fine-structure transitions in Si II. The line strengths in LS coupling are obtained in an ab initio manner in the close-coupling approximation employing the R-matrix method. The fine-structure components are obtained through algebraic transformations of the LS multiplets. Observed spectroscopic energies are employed whenever available. A 12-state eigenfunction expansion of the core ion, Si III, is employed for the present calculations. This work presents the oscillator strengths of 1122 fine-structure transitions in Si II corresponding to 390 LS multiplets and provides a reasonably complete set of radiative transitions for this astrophysically important ion for the first time. Present results are of comparable accuracy to previous detailed calculations obtained for a small number of transitions and are in reasonably good agreement with the measured oscillator strengths and lifetimes.I thank Professor Anil K. Pradhan for comments and suggestions. This work was supported by NASA Grants NAGW-3315 and NAS-32643. The computational work was carried out on the Cray Y-MP at the Ohio Supercomputer Center

Electron-Ion Recombination Rate Coefficients and Photoionization Cross Sections for Astrophysically Abundant Elements. XI. N V-VI and F VII-VIII for Ultraviolet and X-Ray Modeling

Relevant data is available at: http://www.astronomy.ohio-state.edu/~nahar/nahar_radiativeatomicdata/index.htmlThe inverse processes of photoionization and electron-ion recombination for hv + N V ↔ N VI + e, hv + N VI ↔ N VII + e, hv + F VII ↔ F VIII + e, and hv + F VIII ↔ F IX + e are studied in detail using a self-consistent unified method for the total electron-ion recombination. The method enables calculation of the total and level-specific recombination rate coefficients α_R and α_R(i), subsuming both radiative and dielectronic recombination (RR and DR). The photoionization and recombination cross sections σ_PI and σ_RC are computed using an identical wave function expansion for both processes in the close coupling approximation using the R-matrix method. The results include total and partial photoionization cross sections and recombination rate coefficients for all fine-structure levels up to n ≤ 10, about 100 for Li-like N V and F VII with 1/2 ≤ J ≤ 17/2, and over 170 for He-like N VI and F VIII with 0 ≤ J ≤ 10. Level-specific σ_PI(nSLJ ) and α_R(T; nSLJ ) are calculated for the first time for these ions. The coupled-channel wave function expansions for N V and F VII consist of 17 levels of cores N VI and F VIII, respectively, and for N VI and F VIII consist of 16 levels of cores N VII and F IX, respectively. Relativistic fine structure is considered through the Breit-Pauli R-matrix method. The single-valued total α_R(T) is presented over an extended temperature range for astrophysical and laboratory plasma applications. Although the total unified α_R(T) for all ions agree well with the available published RR+DR rates, significant differences are noted at the DR peak for N v. Total α_RC(E) and α_R(E) as functions of photoelectron energy are presented for comparison with experiments. Total rates for H-like N VII and F IX are also given for completeness. The cross sections σ_PI and σ_RC include important atomic effects such as radiation damping, channel couplings, and interference of DR and RR, and should be accurate to within 10%-15%. The comprehensive data sets are applicable for ionization balance and recombination-cascade models for UV and X-ray lines.This work was supported partially by the NASA Astrophysical Theory Program and the Space Astrophysical Research and Analysis programs

Positronium formation during scattering of positrons by hydrogen atoms

Relevant data is available at: http://www.astronomy.ohio-state.edu/~nahar/nahar_radiativeatomicdata/index.htmlThe formation of positronium (Ps) in different states by the impact of intermediate energy (20-500 eV) positrons on atomic hydrogen is calculated using the distorted-wave Born approximation along with the first Born approximation. Differential and integrated cross sections for the formation of Ps(ls), Ps(2s), Ps(2p_0), and Ps(2p_1) have been evaluated individually and then 1/n^3 behavior for charge-transfer cross sections is used for n ≥ 3 to obtain the total cross sections for positronium formation. The present results for positronium formation cross sections are in agreement with the results of other available investigations

Photoionization and electron-ion recombination of Cr I

Using the unified method, the inverse processes of photoionization and electron-ion recombination are studied in detail for neutral chromium, (Cr I + hv ↔ Cr II + e), for the ground and excited states. The unified method based on close-coupling approximation and R-matrix method (i) subsumes both the radiative recombination (RR) and dielectronic recombination (DR) for the total rate and (ii) provides self-consistent sets of photoionization cross sections σ_PI and recombination rates α_RC. The present results show in total photoionization of the ground and excited states an enhancement in the background at the first excited threshold, 3d^4 4s ^5D state of the core. One prominent phot-excitation-of-core (PEC) resonance due to one dipole allowed transition (^6S-^6P^0) in the core is found in the photoionization cross sections of most of the valence electron excited states. Structures in the total and partial photoionization, for ionization into various excited core states and ground state only, respectively, are demonstrated. Results are presented for the septet and quintet states with n ≤ 10 and l ≤ 9 of Cr I. These states couple to the core ground state ^6S and contribute to the recombination rates. State-specific recombination rates are also presented for these states and their features are illustrated. The total recombination rate shows two DR peaks, one at a relatively low temperature, at 630 K, and the other around 40,000 K. This can explain existence of neutral Cr in interstellar medium. Calculations were carried out in LS coupling using a close-coupling wave function expansion of 40 core states. The results illustrate the features in the radiative processes of Cr I and provide photoionization cross sections and recombination rates with good approximation for this astrophysically important ion.This work was partially supported by the NASA Astronomy and Physics Research Analysis Program. The computational work was carried out on Cray machines at the Ohio Supercomputer Center in Columbus Ohio

Oscillator strengths for dipole-allowed fine-structure transitions in Fe XIII

Oscillator strengths, line strengths, and transition probabilities for fine-structure levels in silicon-like iron, Fe XIII, are reported. The data obtained are for 1223 LS bound terms, 64,456 LS multiplets, and 307,863 fine-structure transitions. Calculations are carried out in LS coupling using the close coupling R-matrix approximation with a 14-term eigenfunction expansion. The fine-structure components are obtained through algebraic transformations. Present data considerably exceed the observed and the previously calculated data available, including those from the Opacity Project. Comparisons with previously measured and calculated values are made.I thank Professor Anil K. Pradhan for comments. This work was partially supported by a grant from the NSF for the Iron Project and by the NASA UV and Optical Astrophysical Program. Computations were carried out on the Cray T-94 at the Ohio Supercomputer Center

Electron-ion recombination of Fe II

Relevant data is available at: http://www.astronomy.ohio-state.edu/~nahar/nahar_radiativeatomicdata/index.htmlA complete treatment of electron-ion recombination of e + Fe III → Fe II employing a unified method is presented. The treatment incorporates both the radiative and dielectronic recombinations in a self-consistent manner. Total recombination rate coefficients are obtained from photoionization cross sections, and from collision strengths for dielectronic recombination calculated using the precise theory of Bell and Seaton [J. Phys. B 18, 1589 (1985)]. Large-scale computations for both of these quantities are carried out in the close coupling approximation using the R-matrix method with an eigenfunction expansion that includes 83 states of Fe III dominated by the ground 3d^6, and the excited 3d^5 4s and 3d^5 4p configurations. Both the total and state-specific recombination rate coefficients are obtained. Comparison of the present results with the previous ones shows considerable difference for most of the temperature regions. The present results provide accurate and self-consistent recombination rates, in the temperature range of practical applications (T < 10^5 K), for ionization balance in photoionization models employing the detailed photoionization cross sections from the Opacity Project. [S1050-2947(97)00103-0]This work was supported by NASA Grants No. NAGW-3315 and No. NAS-32643

Atomic data from the Iron Project LIV. Relativistic calculations for allowed and forbidden fine structure transitions in Fe XX

Relevant data is available at: http://www.astronomy.ohio-state.edu/~nahar/nahar_radiativeatomicdata/index.htmlAn extensive set of oscillator strengths (f), line strengths (S) and radiative decay rates (A) for dipole allowed, intercombination and forbidden transitions in Fe XX is presented. Results include 1792 bound fine structure levels of total angular momenta J = 1/2–19/2 of even and odd parities, with 2 ≤ n ≤ 10, 0 ≤ l ≤ 14, orbital angular momenta, 0 ≤ L ≤ 14, and total spin multiplicities 2S+1 = 2,4,6, yielding to about 3.8 × 10^5 allowed and 13,874 forbidden transitions in Fe XX. These results far exceed the available data in literature. The existing data compiled by the National Institute for Standards and Technology (NIST) are available for little over one hundred transitions, and the previously calculated data under the Opacity Project includes 75 590 LS multiplets. The present ab initio calculations are carried out in the close coupling approximation using the relativistic Breit-Pauli R-matrix method for allowed and intercombination E1 transitions. The wavefunction expansion includes 20 fine structure levels of configurations 2s^2 2p^2, 2s2p^3, and 2p^4 of the core ion Fe XXI. Computed energy levels are identified spectroscopically using a newly developed procedure based on quantum defects and channel contributions. All 55 observed levels have been identified and are in agreement to much less than 1% with most of the calculated values. Coefficients for radiative decays for the forbidden E2, E3, M1 transitions are obtained through atomic structure calculations including relativistic terms in the Breit-Pauli approximation using the code SUPERSTRUCTURE. The theoretical transition probabilities are compared with available values, with varying degree of agreement.This work was partially supported by US National Science Foundation (AST-9870089) and the NASA ADP program