Measurement and interpretation of the polarization of the x-ray line emission of heliumlike FeXXV excited by an electron beam

The linear polarization of the 1s2p1P1→1s21S0 resonance line, the 1s2p3P1,2→1s21S0 intercombination lines, and the 1s2s3S1→1s21S0 forbidden line was measured in heliumlike FeXXV excited near threshold by a monoenergetic electron beam. The measurement was carried out with a high-resolution x-ray spectrometer employing a set of two analyzing crystals that acted as polarizers by selectively reflecting the individual polarization components. A value of +0.56+0.17-0.08 was determined for the polarization of the 1P1 line, -0.53+0.05-0.02 for the 3P2 line, -0.22+0.05-0.02 for the 3P1 line, and -0.076+0.007-0.007 for the 3S1 line. The measurements were compared with results from a relativistic distorted-wave calculation, which was carried out for a number of mid-Z heliumlike ions (Mg10+endash Kr34+), and good agreement was found. By contrast, disagreement was noted with predictions based on Coulomb-Born calculations, allowing us to distinguish between theoretical approaches. copyright 1996 The American Physical Society