Hyperfine-induced effects on the K alpha 1 angular distribution following electron-impact excitation of heliumlike spin-1/2 Tl79+ions

Angular distribution of the K alpha 1 (1s2p 1,3P1,2 1s2 1S0) emission line following electron-impact excitation of heliumlike spin-1/2 Tl79+ ions is studied within the framework of the density-matrix theory and the relativistic distorted-wave theory. In particular, we aim to explore how a (nonzero) nuclear magnetic dipole moment mu I affects the K alpha 1 angular distribution due to the hyperfine interactions. To this end, detailed calculations are performed for selected spin-1/2 A81Tl79+ (A = 187, 205, and 207) ions with large mu I. It is found that the hyperfine-induced effects on the K alpha 1 angular distribution depend strongly on the impact electron energy. When compared with the case of zero-spin Tl79+ ions, the K alpha 1 angular distribution becomes much more anisotropic for low impact energies, whereas this anisotropy decreases quickly at intermediate and high impact energies. Such a behavior is in sharp contrast to the results for the fine-structure-resolved 1s2p 3P2 1s2 1S0 component [Phys. Rev. A 102, 042813 (2020)]. Therefore, accurate K alpha 1 angular measurements at low impact electron energies could be employed as a sensitive tool to probe the hyperfine interaction in highly charged few-electron ions.