Resolution of the forbidden (J = 0 → 0) excitation puzzle in Mg-like ions

¹ Department of Physics, University of Strathclyde, Glasgow G4 0NG, UK
e-mail: luis.fernandez-menchero@strath.ac.uk
² Department of Applied Mathematics and Theoretical Physics, University of Cambridge, Cambridge CB3 0WA, UK

Received: 21 January 2015
Accepted: 8 April 2015

Abstract

We investigate the source of the discrepancy between R-matrix and distorted-wave (DW) collision strengths for J − J′ = 0–0 transitions in Mg-like ions, for example 3s²¹S₀–3p²¹S₀, as reported previously. We find it to be due to the neglect of coupling, for example via 3s3p ¹P₁, as done by most DW codes. We have implemented an option to account for such coupling as a perturbation within the autostructure DW code. This removes the discrepancy of a factor ~10 and ~100 for Fe^{14 +} and S^{4 +}, respectively, for such transitions. The neglect of coupling would have affected (to some degree) the atomic data for a few weak optically forbidden transitions in other isoelectronic sequences if they were calculated with DW codes such as FAC and HULLAC. In addition, we compare the Fe^{14 +} line intensities predicted with the R-matrix collision strengths against observations of solar active regions and flares; they agree well. For Fe^{14 +}, we suggest that the best density diagnostic ratio is 327.0/321.8 Å.