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

The linear polarization of the 1s2p P-1(1)-->1s(2 1)S(0) resonance line, the 1s2p P-3(1,2)-->1s(2 1)S(0) intercombination lines, and the 1s2s S-3(1)-->1s(2 1)S(0) forbidden line was measured in heliumlike Fe XXV 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(-0.08)(+0.17) was determined for the polarization of the P-1(1) line,-0.53(-0.02)(+0.05) for the P-3(2) line, -0.22(-0.02)(+0.05) for the P-3(1) line, and -0.076(-0.007)(+0.007) for the S-3(1) line. The measurments were compared with results from a relativistic distorted-wave calculation, which was carried out for a number of mid-Z heliumlike ions (Mg10+-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.