Angular correlations in the two-photon decay of aligned hydrogenlike ions

The two-photon decay of highly charged hydrogenlike ions is studied within the framework of second-order perturbation theory, based on Dirac's equation. Attention has been paid to the angle-angle correlations between the two emitted photons. In particular, the density matrix theory is applied in order to explore how these angle-angle correlations are affected by the alignment of the excited states of the ions. Detailed computations have been carried out for the 3d(5/2)-> 1s(1/2) transition in hydrogenlike uranium U91+ ions for different (degrees of) initial alignment. It is shown that the angular correlation function is very sensitive to the alignment of the 3d(5/2) state, an effect which has to be accounted for in the analysis of all (future) two-photon decay experiments, for which the excited state of the ions is produced by relativistic ion-atom collisions.