Effect of the Breit interaction on spin polarization of Auger electrons following electron-impact excitation of Be-like ions

Abstract

Spin polarization of Auger electrons emitted from nonradiative Auger decay \(1s2s^{2}2p_{1/2}\,J\!=\!1 \!\rightarrow \! 1s^{2}2s\,J_{f}\!=\!1/2\) following inner-shell electron-impact excitation from the ground state of highly charged Be-like ions is studied by using the density-matrix theory and the relativistic distorted-wave theory. The question is raised: How does the Breit interaction affect spin polarization of the emitted Auger electrons? Generally, both alignment and intrinsic anisotropy parameters determine the anisotropy of the emitted electrons in this excitation–autoionization process. Here, we calculate and analyze intrinsic spin-polarization parameter for Be-like \(\hbox {Mg}^{8+}\), \(\hbox {Fe}^{22+}\), \(\hbox {Mo}^{38+}\), \(\hbox {Nd}^{56+}\), \(\hbox {Au}^{75+}\), and \(\hbox {U}^{88+}\) ions to obtain spin polarization of the Auger electrons. It is found that the Breit interaction hardly contributes to the spin polarization for low-Z Be-like ions such as \(\hbox {Mg}^{8+}\), even at high impact electron energies, whereas for medium- and high-Z ions the situation becomes fairly different. To be specific, the spin polarization becomes nearly zero and almost independent of the emission angle of the Auger electrons owing to the Breit interaction, which becomes more and more prominent with increasing nuclear charge. Moreover, the Breit interaction even changes the direction of the spin polarization vector for high-Z ions especially at low impact energies, for example, for \(\hbox {U}^{88+}\) ions at impact energies below the threefold excitation threshold.