Angular correlations in the two-photon decay of hydrogenlike ions: Relativistic Green’s-function approach

The angular correlations in the two-photon decay of hydrogenlike ions are studied within the framework of second-order perturbation theory, based on Dirac's equation. Particular attention has been paid to the effects which arise from the higher (nondipole) terms in the expansion of the electron-photon interaction. It is shown that the photon-photon angular correlation function, which is found symmetric with respect to the angle θ=90^∘ in the electric dipole approximation, becomes asymmetric because of the nondipole contributions, and that this effect is enhanced as the nuclear charge Z increases. Detailed computations on the photon-photon angular distribution have been carried out for the 2s1∕2→1s1∕2 and 3d5∕2→1s1∕2 transitions in neutral hydrogen (H) as well as for hydrogenlike xenon (Xe⁵³⁺) and uranium (U⁹¹⁺) ions, and are compared with previous nonrelativistic results by Au [Phys. Rev. A 14, 531 (1976)]).