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

Abstract

The linear polarization of the 1s2p${\mathrm{}}^{1}$${\mathit{P}}_{1}$\ensuremath{\rightarrow}1${\mathit{s}}^{2}$${\mathrm{}}^{1}$${\mathit{S}}_{0}$ resonance line, the 1s2p${\mathrm{}}^{3}$${\mathit{P}}_{1,2}$\ensuremath{\rightarrow}1${\mathit{s}}^{2}$${\mathrm{}}^{1}$${\mathit{S}}_{0}$ intercombination lines, and the 1s2s${\mathrm{}}^{3}$${\mathit{S}}_{1}$\ensuremath{\rightarrow}1${\mathit{s}}^{2}$${\mathrm{}}^{1}$${\mathit{S}}_{0}$ forbidden line was measured in heliumlike Fe XXV excited near threshold by a monoenergetic electron beam. The measurement was carried out with a high-resolution x-ray spectrometer employing a set of two analyzing crystals that acted as polarizers by selectively reflecting the individual polarization components. A value of +0.${56}_{\mathrm{\ensuremath{-}}0.08}^{+0.17}$ was determined for the polarization of the $^{1}$${\mathit{P}}_{1}$ line, -0.${53}_{\mathrm{\ensuremath{-}}0.02}^{+0.05}$ for the $^{3}$${\mathit{P}}_{2}$ line, -0.${22}_{\mathrm{\ensuremath{-}}0.02}^{+0.05}$ for the $^{3}$${\mathit{P}}_{1}$ line, and -0.${076}_{\mathrm{\ensuremath{-}}0.007}^{+0.007}$ 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. \textcopyright{} 1996 The American Physical Society.