Breit-Pauli R-matrix method for electron-impact excitation to magnetic sublevels and x-ray-line polarization of ions

The Breit-Pauli R-matrix (BPRM) method was employed to calculate x-ray line polarization resulting from electron-impact excitation to excited-state magnetic sublevels (MS) of complex ions. In the development of this work, the calculation of MS electron-impact excitation cross sections in the present BPRM method is computationally efficient when the angular integration of the scattering polar angle is performed in an analytical manner. The BPRM method is then applied to calculate the line polarization of 3C (lambda = 15.01 angstrom) and 3D(lambda = 15.26 angstrom), two well-known x-ray lines of 17 (an ion with a medium nuclear charge and ion charge). We demonstrate in this work the resonance effects in MS cross sections, which contribute to the energy dependence in the polarization. The BPRM calculations of Fe XVII are further compared with the results calculated by the Dirac R-matrix (DRM) method. Generally, most of the results calculated from the BPRM method and the DRM method are in good agreement. This comparison confirms the earlier DRM calculations of resonance effects in MS electron-impact excitation cross sections of 3C and 3D, and thus demonstrates the applicability of the BPRM method to the calculation of x-ray line polarization of 3C and 3D, important for the analysis of x-ray spectra observed on an electron beam ion trap.