Effect of the Breit interaction on the angular distribution of Auger electrons following electron-impact excitation of highly charged Be-like ions

Abstract

The electron-impact excitation from the ground state to the autoionizing level $1s2{s}^{2}2{p}_{1/2}\phantom{\rule{0.16em}{0ex}}J=1$ and the subsequent nonradiative Auger decay $1s2{s}^{2}2{p}_{1/2}\phantom{\rule{0.16em}{0ex}}J=1\ensuremath{\rightarrow}1{s}^{2}2s\phantom{\rule{0.16em}{0ex}}J=1/2$ of Be-like ions have been studied by using the multiconfigurational Dirac-Fock method and the relativistic distorted-wave theory. Special attention has been paid to the effect of the Breit interaction on the angular distribution of the Auger electrons emitted. To do so, the partial cross sections, alignment parameters of the autoionizing level, and intrinsic anisotropy parameters of the Auger decay are evaluated for Be-like ${\mathrm{Mg}}^{8+}, {\mathrm{Fe}}^{22+}, {\mathrm{Mo}}^{38+}, {\mathrm{Nd}}^{56+}, {\mathrm{Au}}^{75+}$, and ${\mathrm{U}}^{88+}$ ions, from which the angular distribution of the Auger electrons is further obtained. It is found that the Breit interaction hardly contributes to the angular distribution for low-$Z$ ions such as ${\mathrm{Mg}}^{8+}$, especially at low impact energies, while for medium- and high-$Z$ ions the situation becomes fairly different. To be specific, the Breit interaction contributes to lowering the anisotropy of the angular distribution, which first becomes quickly more prominent with increasing atomic number up to certain high-$Z$ ions and then behaves very slowly less prominently for higher-$Z$ ions. Moreover, for high-$Z$ ions the effect of the Breit interaction on the angular distribution is found to be nearly independent of the atomic number $Z$. Owing to opposite effects of the Breit interaction on the alignment parameters and the intrinsic anisotropy parameters, such an ``abnormal behavior'' is speculated to be caused very likely by a ``competition'' between the opposite effects of the Breit interaction on both of them.