Angular and polarization properties of the Lyman- α1 line 2p3/2→1s1/2 following electron-impact excitation of hydrogenlike ions

Abstract

Electron-impact excitation from the ground state to the excited energy level $2{p}_{3/2}$ of hydrogenlike ions and subsequent Lyman-${\ensuremath{\alpha}}_{1}$ $(2{p}_{3/2}\ensuremath{\rightarrow}1{s}_{1/2})$ radiative decay are investigated using the relativistic distorted-wave method. Special attention is paid to the linear polarization and angular distribution of the Lyman-${\ensuremath{\alpha}}_{1}$ line and also to the effects of the Breit interaction. To this aim, detailed calculations are performed for hydrogenlike ${\mathrm{Ti}}^{21+}$, ${\mathrm{Mo}}^{41+}$, ${\mathrm{Ba}}^{55+}$, and ${\mathrm{Au}}^{78+}$ ions. It is found that the presently obtained (partial cross sections and) linear polarization agree excellently with other theoretical and experimental results available for low-$Z\phantom{\rule{4pt}{0ex}}{\mathrm{Ti}}^{21+}$ ions within the experimental uncertainties. Moreover, the Lyman-${\ensuremath{\alpha}}_{1}$ line is found to be less linearly polarized and less anisotropic due to the contribution of the Breit interaction. Such effects of the Breit interaction behave more prominently for higher-$Z$ ions and higher impact electron energies, respectively. For instance, the Breit interaction qualitatively changes the polarization behavior and angular emission pattern of the Lyman-${\ensuremath{\alpha}}_{1}$ line from high-$Z\phantom{\rule{4pt}{0ex}}{\mathrm{Au}}^{78+}$ ions at the impact energy of about 4.2 times the corresponding excitation threshold.