Circular polarization of the Fe XXV x-ray lines following collisional excitation by longitudinally polarized electrons.

Abstract

The circular polarization of x-ray lines emitted by heliumlike iron collisionally excited by a longitudinally polarized electron beam has been investigated theoretically. The required collision strengths for excitation of the different n=2 and 3 magnetic sublevels from the 1${\mathit{s}}^{2}$ ground level have been calculated by two different distorted-wave methods and compared with each other. The agreement is generally good. One of these methods is semirelativistic, and the other is fully relativistic with QED effects included in the atomic structure part of the calculations. A high degree of circular polarization is obtained for the magnetic dipole 1 $^{1}$${\mathit{S}}_{0--}$2 $^{3}$${\mathit{S}}_{1}$ and quadrupole 1 $^{1}$${\mathit{S}}_{0--}$2 $^{3}$${\mathit{P}}_{2}$ lines, while the intercombination line 1 $^{1}$${\mathit{S}}_{0--}$2 $^{3}$${\mathit{P}}_{1}$ is found to be almost completely circularly polarized in the near-threshold region. In addition to being of intrinsic interest for atomic collision physics, the reported results might have useful applications in future diagnostics of high-temperature plasmas with a view to providing information on the possible polarization of suprathermal electron beams.