Polarization of M 2 line emitted from highly-charged beryllium-like ions following electron-impact excitation

The contributions of the Breit interaction to the linear polarization of the 1s2s 2 2p 3/2 J =2 → 1s 2 2s 2 J =0 magnetic quadrupole (M2) line following electron-impact excitation have been 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 M 2 fluorescence radiation and that these depolarization effects increase as the incident electron energy and/or the atomic number is enlarged.