Angular correlations in the two-photon decay of heliumlike 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) nondipole 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 S-1(0), 1s2s S-3(1), and 1s2p P-3(0) states of helium-like Xe52+, Au77+, and U90+ ions.