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 1s2p¹{P}₁→1{s}²¹{S}₀$ resonance line, the 1s2p${}³P1,2{→}1s²{}¹S₀ intercombination lines, and the 1s2s³{S}₁→1{s}²¹{S}₀$ 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 ¹{P}₁$ line, -0.${53}_{{{-}}0.02}+0.05$ for the $³P₂ line, -0.22_-0.02+0.05 for the ³{P}₁$ 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⁺--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. {} 1996 The American Physical Society.