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${\mathrm{}}^1$${\mathit{P}}_1$→1${\mathit{s}}^2$${\mathrm{}}^1$${\mathit{S}}_0$ resonance line, the 1s2p${\mathrm{}}^3$${\mathit{P}}_{1,2}$→1${\mathit{s}}^2$${\mathrm{}}^1$${\mathit{S}}_0$ intercombination lines, and the 1s2s${\mathrm{}}^3$${\mathit{S}}_1$→1${\mathit{s}}^2$${\mathrm{}}^1$${\mathit{S}}_0$ forbidden line was measured in heliumlike Fe XXV excited near threshold by a monoenergetic electron beam. The measurement was carried out with a high-resolution x-ray spectrometer employing a set of two analyzing crystals that acted as polarizers by selectively reflecting the individual polarization components. A value of +0.${56}_{\mathrm{-}0.08}^{+0.17\mathrm{}}$ was determined for the polarization of the ${}_{}{}^1{}_{}{}^{}$${\mathit{P}}_1$ line, -0.${53}_{\mathrm{-}0.02}^{+0.05\mathrm{}}$ for the ${}_{}{}^3{}_{}{}^{}$${\mathit{P}}_2$ line, -0.${22}_{\mathrm{-}0.02}^{+0.05\mathrm{}}$ for the ${}_{}{}^3{}_{}{}^{}$${\mathit{P}}_1$ line, and -0.${076}_{\mathrm{-}0.007}^{+0.007\mathrm{}}$ for the ${}_{}{}^3{}_{}{}^{}$${\mathit{S}}_1$ 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 (${\mathrm{Mg}}^{10+}$–${\mathrm{Kr}}^{34+}$), 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.