Electron impact excitation of Cl XVI

K. M. Aggarwal and F. P. Keenan (Astrophysics Research Centre, School of Mathematics and Physics, Queen’s University, Belfast BT7 1NN, Northern Ireland, UK)

Emission lines from several ionisation stages of chlorine, including He-like Cl XVI, have been observed in solar plasmas – see, for example, Sylwester et al. [1]. The n = 2 lines of He-like ions in the x-ray region (namely, the resonance w: 1s2 1S0 – 1s2p 1P1, intercombination x and y: 1s2 1S0 – 1s2p 3P2,1, and forbidden z: 1s2 1S0 – 1s2p 3S1) are of
particular interest as these are useful for the determination of electron densities and temperatures in the solar corona and transition region [1,2]. Emission lines of Cl XVI have also been measured in lasing [3] and fusion plasmas [2]. However, to analyse observations, atomic data are required for a variety of parameters, such as energy levels, radiative rates (A- values), and excitation rates or equivalently the effective collision strengths (Y), which are obtained from the electron impact collision strengths (Ω). Additionally, atomic data for Cl XVI are highly required for the study and modelling of fusion plasmas.

Experimentally, only energy levels are available for Cl XVI on the NIST website. Similarly, A-values are also available for some transitions on the NIST website, but there is paucity for accurate collisional atomic data for Cl XVI, as Y values available in the literature [4] are
interpolated from the data for other He-like ions. Therefore, in this paper we report a complete set of results (namely energy levels, radiative rates, and effective collision strengths) for all transitions among the lowest 49 levels of Cl XVI. These levels belong to the 1s2, 1s2l, 1s3l, 1s4l, and 1s5l configurations. Finally, we also report the A-values
for four types of transitions, namely electric dipole (E1), electric quadrupole (E2), magnetic dipole (M1), and magnetic quadrupole (M2), because these are also required for plasma modelling.

For the determination of wavefunctions we employ the fully relativistic GRASP code, and for the calculations of Ω, the Dirac atomic R-matrix code (DARC) of PH Norrington and IP Grant. Collision strengths and effective collision strengths are calculated for all 1176 transitions among the 49 levels of the n ≤ 5 configurations over a wide energy (temperature) range up to 580 Ryd (107.2 K), suitable for applications in astrophysical and other plasmas. Additionally, parallel calculations have also been performed with the Flexible Atomic Code (FAC) of Gu [5], so that all atomic parameters can be rigorously assessed for accuracy. Detailed data along with comparisons will be presented during the conference.

Poster presented at the 17th International Conference on Atomic Processes in Plasmas, Queen’s University Belfast, 19-22 July 2011.


  1. J. Sylwester et al., Ad. Space Res. 42, 838 (2008)
  2. I. H. Coffey et al., Phys. Scr. 47, 169 (1993)
  3. V. V. Gavrilov et al., Quant. Elect. 31, 1071 (2001)
  4. F. P. Keenan et al., Phys. Scr. 35, 432 (1987)
  5. M. F. Gu, ApJ 582, 1241 (2003)
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