Shift of Zeeman Components of Ionic Spectral Lines Caused by the Angular Doppler Effect

The angular Doppler effect was introduced by Garetz. While the usual Doppler effect is caused by the translational motion of the emitter, the angular Doppler effect is caused by the rotational motion of the emitter. It results into the frequency shift of a circularly-polarized light. In the present paper we analyze the angular Doppler effect caused by the rotational motion of ions in a magnetic field. We demonstrate that for the spectral lines emitted by ions, this effect results in the shift of the σ-components of the Zeeman triplet observed along the magnetic field. We compare the angular Doppler shift in this situation with the fine structure splitting. We show (by the example of He+) that the angular Doppler shift can exceed the fine structure splitting at sub-Giga Gauss magnetic fields. Magnetic fields of this magnitude and even much higher were theoretically expected to develop and were indeed measured in experiments studying relativistic laser-plasma interactions. Magnetic fields of this magnitude and even much higher occur in neutron stars, some of them having helium in their tiny atmospheres. These are the physical situations relevant to our findings.