Influence of Breit interaction on the polarization of radiation following inner-shell electron-impact excitation of highly charged berylliumlike ions

Electron-impact excitation cross sections from the ground state to the individual magnetic sublevels of the excited state $1s2s^{2}2p_{1/2}J=1$ for highly charged berylliumlike ions have been calculated by using a fully relativistic distorted-wave (RDW) method. The degrees of linear polarization of the corresponding radiations are obtained. It is found that the Breit interaction makes the linear polarization of radiations decrease, and this character becomes more evident with increasing of incident electron energy. For a given energy in threshold units, the linear polarization without the Breit interaction included increases very slowly as the atomic number increases, however, the linear polarization with the Breit interaction included decreases rapidly. These characteristics are very different from the conclusions for the linear polarization of the same radiations but formed by the dielectronic recombination process [Phys. Rev. Lett. 103, 113001 (2009)], in which the Breit interaction makes the linear polarization increase, and the linear polarization with the Breit interaction included increases with increasing of the atomic number.

Figure 1

Total electron-impact excitation cross sections (barns) for excitation from the ground state $1s^{2}2s^{2}J=0$ to the excited state $1s2s^{2}2p_{1/2}J=1$ for the berylliumlike Mo${}^{38+}$, Nd${}^{56+}$, and Bi${}^{79+}$ ions as functions of incident electron energy in threshold units. R represents the values with inclusion of only the Coulomb interaction, and RB represents the ones with the Breit interaction included.

Figure 2

Cross sections (barns) for electron-impact excitation from the ground state $1s^{2}2s^{2}J=0$ to the specific magnetic sublevels $m_f=0$ and $m_f=\pm 1$ of the excited state $1s2s^{2}2p_{1/2}J=1$ for the berylliumlike Mo${}^{38+}$, Nd${}^{56+}$, and Bi${}^{79+}$ ions as functions of incident electron energy in threshold units. R represents the values with inclusion of only the Coulomb interaction, and RB represents the ones with the Breit interaction included.

Figure 3

The degree of linear polarization of the transition line $1s2s^{2}2p_{1/2}J=1$$\to$$1s^{2}2s^{2}J=0$ for the berylliumlike Mo${}^{38+}$, Nd${}^{56+}$, and Bi${}^{79+}$ ions as functions of incident electron energy in threshold units. R represents the values with inclusion of only the Coulomb interaction, and RB represents the ones with the Breit interaction included.

Figure 4

The degree of linear polarization of the transition line $1s2s^{2}2p_{1/2}J=1$$\to$$1s^{2}2s^{2}J=0$ for the berylliumlike ions as functions of atomic number at incident electron energy is four times of threshold energy. R represents the values with inclusion of only the Coulomb interaction, and RB represents the ones with the Breit interaction included.