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Angular correlations in the two--photon decay of helium--like heavy ions

The two-photon decay of heavy, helium-like ions is investigated based on second-order perturbation theory and Dirac's relativistic equation. Special attention has been paid to the angular emission of the two photons, i.e., how the angular correlation function depends on the shell structure of the ions in their initial and final states. Moreover, the effects from the (electric and magnetic) non-dipole terms in the expansion of the electron-photon interaction are discussed. Detailed calculations have been carried out for the two-photon decay of the $1s2s \, ^1S_0$, $1s2s \, ^3S_1$ and $1s2p \, ^3P_0$ states of helium-like xenon Xe$^{52+}$, gold Au$^{77+}$ and uranium U$^{90+}$ ions.