Relativistic effects on the polarization of line radiation emitted from He-like and H-like ions following electron-impact excitation

The radiation emitted from ions excited by a directed electron beam can be strongly linearly polarized. According to Itikawa, Srivastava, and Sakimoto [Phys. Rev. A. 44, 7195 (1991)] the degree of polarization should be independent of atomic number in an isoelectronic sequence when expressed as a function of incident-electron energy in threshold units. We used a distorted-wave computer code to calculate cross sections for electron-impact excitation to specific magnetic sublevels of H- and He-like ions with Z=13, 18, 22, 42, 56, 79, and 92. We found that the polarization of the resulting radiation is indeed independent of atomic number in the nonrelativistic limit. But when relativistic effects are properly taken into account, the polarization is markedly Z dependent. We show how the differences between the relativistic and nonrelativistic results vary with the incident-electron energy, as well as with increasing atomic number.