Comment on 'Dirac R-matrix method for the calculation of x-ray line polarization'
In a recent article by Chen et al. [Phys. Rev. A 79, 062715 (2009)], reference is made to magnetic sublevel collision strengths in an earlier relativistic distorted-wave (RDW) article by Zhang et al. [Phys. Rev. A 41, 198 (1990)]. In the former reference, Chen et al. carried out Dirac R-matrix calculations that suggest the polarization of the 3C and 3D lines of Fe xvii, which can be computed from the magnetic sublevel collision strengths, differ from RDW results by as much as 20%. We have recently carried out a variety of RDW and Dirac R-matrix calculations of the 3C and 3D polarizations that demonstrate this quantity to be relatively insensitive to the size and details of the atomic model. Moreover, the polarizations obtained from these recent RDW and Dirac R-matrix calculations agree well, and they also agree well with the polarizations that can be obtained from the fundamental collision data published by Zhang et al. This good agreement between RDW and Dirac R-matrix polarizations contradicts the behavior reported by Chen et al.