We have developed the theory of the polarization of radiation emitted from an aligned ion in any one-photon transition involving two different electromagnetic multipoles. We have applied the theory to the line 1s2p 3P2 → 1s2 1S0, labelled x, emitted from He-like ions Sc19+ and 205Tl79+ which are collisionally excited by an electron beam. In these ions with a non-zero nuclear spin, a mixture of magnetic quadrupole (M2) and hyperfine-induced electric dipole (E1) transitions occurs in the line x, the ratio of the E1 to M2 transition probability being equal to 0.350 for scandium and 0.118 for thallium. Our calculations show that quantum interference between the E1 and M2 channels has the effect of increasing the degree of linear polarization of x, slightly for Sc19+ but substantially for 205Tl79+. For individual hyperfine components of x, the increase of the degree of polarization can be much more significant. On the other hand, the effects of E1–M2 interference are found to reduce the anisotropy of the intensity angular distribution of the line x.

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