Effect of the Breit interaction on inner-shell electron-impact excitation and subsequent radiative decay of highly charged berylliumlike ions

Abstract

The inner-shell electron-impact excitation from the ground state to the $1s2{s}^{2}2{p}_{1/2}\phantom{\rule{0.16em}{0ex}}{J}_{f}=1$ excited state and the subsequent electric-dipole $1s2{s}^{2}2{p}_{1/2}\phantom{\rule{0.16em}{0ex}}{J}_{f}=1\ensuremath{\rightarrow}1{s}^{2}2{s}^{2}\phantom{\rule{0.16em}{0ex}}{J}_{i}=0$ radiative decay of berylliumlike ions with zero nuclear spin have been studied within the framework of the multiconfigurational Dirac-Fock method and the relativistic distorted-wave theory. Special attention has been paid to answering the question of how the Breit interaction affects such a two-step ``excitation plus decay'' process. To this end, we explore the effect of the Breit interaction on the second-order alignment parameter of the excited state and also the angular distribution of the emitted characteristic photons. It is found that, for low-$Z$ berylliumlike ions such as ${\mathrm{Ne}}^{6+}$, the Breit interaction hardly contributes to the alignment and the angular distribution even at high-impact electron energies. In contrast, the contribution from the Breit interaction for intermediate- and high-$Z$ ions such as ${\mathrm{Mo}}^{38+}, {\mathrm{W}}^{70+}$, and ${\mathrm{U}}^{88+}$ is of the essence, and such a contribution becomes more and more significant with increasing atomic number and electron energy. To be more specific, the Breit interaction even changes qualitatively the relative population of the magnetic substates of the excited state as described by the alignment parameter and the angular emission pattern of the characteristic photons, for example, for ${\mathrm{W}}^{70+}$ and ${\mathrm{U}}^{88+}$ ions at the electron energies beyond 3.5 and 3.3 times the excitation thresholds, respectively.

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