Detailed calculations are carried out for the polarization properties and angular distribution of the x-ray photoemission of highly charged H-like and He-like ions following electron-impact excitation using a fully relativistic distorted-wave method. The contributions of the magnetic quadrupole ($M2$) to the subsequent characteristic x-ray emission are investigated systematically. For the Lyman-${\alpha }_1$ $(2p_{3/2}\to 1s_{1/2}$) radiation of H-like ions, it is shown that the interference between the leading electric dipole ($E1$) decay and the much weaker $M2$ decay may significantly reduce the polarization and slightly increase the angular distribution of the emitted x-rays, these characteristics are very similar to the conclusions regarding the radiative electron capture process [G. Weber et al., Phys. Rev. Lett. 105, 243002 (2010)]. However, for the $2s_{1/2}2p_{3/2}(J=1)\to 1s_{1/2}2s_{1/2}$($J=1$) radiation of He-like ions, the $E1$-$M2$ interference leads to an obvious increase in polarization and a large decrease in the angular distribution of the x-ray photoemission, which is contrary to the Lyman-${\alpha }_1$ line of H-like ions. And all these characters strongly depend on the atomic number and the incident electron energy.

• Received 10 April 2014

DOI:https://doi.org/10.1103/PhysRevA.90.012703