Dominance of the Breit interaction in the cross section and circular polarization of x-ray radiation following longitudinally-polarized-electron-impact excitation of highly charged ions

Longitudinally-polarized-electron-impact excitation cross sections from the ground state to the individual magnetic sublevels of the excited state $1s2s^{2}2p_{3/2}$($J = 2$) of highly charged Be-like ions are calculated using a fully relativistic distorted-wave method. The contributions of the Breit interaction to the cross sections and circular polarizations of the $1s2s^{2}2p_{3/2}$($J = 2$)$\to 1s^{2}2s^2$($J = 0$) magnetic quadrupole (M2) line for selected Be-like ${\mathrm{Ag}}^{43+}$, ${\mathrm{Ho}}^{63+}$, and ${\mathrm{Bi}}^{79+}$ ions are investigated systematically. It is found that the Breit interaction has a large effect and makes the cross sections increase, especially to the $m_f$ = $-1$ and $-2$ sublevels, the Breit interaction can modify the cross sections by several orders of magnitude. These dramatic influences also lead to a remarkable decrease in the circular polarization of subsequent x-ray radiation, the character of which becomes more and more evident with increasing incident energy and atomic number. And all these characteristics are very different from the conclusions for the linear polarization of radiation following the electron-impact process [S. Fritzsche, A. Surzhykov, and T. Stöhlker, Phys. Rev. Lett. 103, 113001 (2009); Z. W. Wu, J. Jiang, and C. Z. Dong, Phys. Rev. A 84, 032713 (2011)].

Figure 1

Total and $m_f$ = 0 magnetic sublevel cross sections for longitudinally-polarized-electron-impact excitation from the ground state $1s^{2}2s^2$($J = 0$) to the excited state $1s2s^{2}2p_{3/2}$($J = 2$) for Be-like ${\mathrm{Ag}}^{43+}$, ${\mathrm{Ho}}^{63+}$, and ${\mathrm{Bi}}^{79+}$ ions as functions of incident polarized electron energy in threshold units. NB represents the values with inclusion of only the Coulomb interaction, and B represents the ones with the Breit interaction included.

Figure 2

Cross sections for longitudinally-polarized-electron-impact excitation from the ground state $1s^{2}2s^2$($J = 0$) to the specific magnetic sublevels $m_f=\pm 2$ and $\pm 1$ of the excited state $1s2s^{2}2p_{3/2}$($J = 2$) for Be-like ${\mathrm{Ag}}^{43+}$, ${\mathrm{Ho}}^{63+}$, and ${\mathrm{Bi}}^{79+}$ ions as functions of incident polarized electron energy in threshold units. NB represents the values with inclusion of only the Coulomb interaction, and B represents the ones with the Breit interaction included.

Figure 3

The degree of circular polarization of the transition line $1s2s^{2}2p_{3/2}$($J = 2$)$\to 1s^{2}2s^2$($J = 0$) for Be-like ${\mathrm{Ag}}^{43+}$, ${\mathrm{Ho}}^{63+}$, and ${\mathrm{Bi}}^{79+}$ ions as functions of incident polarized electron energy in threshold units. NB represents the values with inclusion of only the Coulomb interaction, and B represents the ones with the Breit interaction included.

Figure 4

The degree of circular polarization of the transition line $1s2s^{2}2p_{3/2}$($J = 2$)$\to 1s^{2}2s^2$($J = 0$) for Be-like ions as functions of atomic number at incident polarized electron energy is four times the threshold energy. NB represents the values with inclusion of only the Coulomb interaction, and B represents the ones with the Breit interaction included.