Dirac R-matrix calculation for electron-impact excitation of S xiii

¹ Key Laboratory of Optical Astronomy, National Astronomical Observatories, Chinese Academy of Sciences, 100012 Beijing, PR China
e-mail: [gyliang;gzhao]@bao.ac.cn
² Pakistan Atomic Energy Commission, PO Box 1114, 44000 Islamabad, Pakistan

Received: 2 May 2013
Accepted: 28 May 2013

Abstract

Context. Sulfur emission lines in the soft X-ray and extreme-ultraviolet regions are observed in a variety of laboratory and astrophysical spectra. But accurate electron impact excitation data for S xiii for state-of-the-art NLTE spectral models are scarce.

Aims. We calculated electron-impact excitation collision strengths and effective collision strengths of S xiii for transitions among the lowest-lying 98 fine-structure states 1s²2lnl′ corresponding to principal quantum numbers n = 2,3,4. The effective collision strengths for these transitions were computed over a wide temperature range (log ₁₀T_e  (K) = 4.53–7.53) for various astrophysical plasma conditions.

Methods. We used the fully-relativistic parallel Dirac R-matrix code to calculate collision strengths. To generate target wavefunctions and energy levels for scattering calculations, we employed the GRASP0 multi-configuration Dirac-Fock code for states up to n = 5.

Results. The wavefunctions are generated from 27 configurations – 1s²2lnl′(n = 2,3,4,5) – giving rise to 166 jj energy levels. The collision and effective collision strengths among the lowest 98 fine-structure levels are compared with the previous theoretical calculations. The collision strengths for most transitions agree well at higher incident electron energies.

Conclusions. The resonant contributions to effective collision strengths are most dominant at lower temperatures.