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 ca n be strongly linearly polarized. According to Itikawa, Srivastava, an d Sakimoto [Phys. Rev. A. 44, 7195 (1991)] the degree of polarization should be independent of atomic number in an isoelectronic sequence wh en expressed as a function of incident-electron energy in threshold un its. We used a distorted-wave computer code to calculate cross section s 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 tha t the polarization of the resulting radiation is indeed independent of atomic number in the nonrelativistic limit. But when relativistic eff ects are properly taken into account, the polarization is markedly Z d ependent. We show how the differences between the relativistic and non relativistic results vary with the incident-electron energy, as well a s with increasing atomic number.