Measurement and interpretation of the polarization of the x-ray line emission of heliumlike Fe

The linear polarization of the 1s2p P1→1s S0 resonance line, the 1s2p P1,2→1s S0 intercombination lines, and the 1s2s S1→1s S0 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 10.5620.08 10.17 was determined for the polarization of the P1 line, 20.5320.02 10.05 for the P2 line, 20.2220.02 10.05 for the P1 line, and 20.07620.007 10.007 for the S1 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 ~Mg–Kr!, and good agreement was found. By contrast, disagreement was noted with predictions based on Coulomb-Born calculations, allowing us to distinguish between theoretical approaches. @S1050-2947~96!05406-6#