Polarization of M2 Line Emitted Following Electron-Impact Excitation of Beryllium-Like Ions

Detailed calculations are carried out for the electron-impact excitation cross sections from the ground state to the individual magnetic sublevels of the 1s2s²2p_3/2 J=2 excited state of highly-charged beryllium-like ions by using a fully relativistic distorted-wave (RDW) method. The contributions of the Breit interaction to the linear polarization of the 1s2s²2p_3/2 J=2→1s²2s² J=0 magnetic quadrupole (M2) line are investigated systematically for the beryllium isoelectronic sequence with 42≤Z ≤92. It is found that the Breit interaction depolarizes significantly the linear polarization of the M2 fluorescence radiation and that these depolarization effects increase as the incident electron energy and/or the atomic number is enlarged.