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 $1s2s\hspace{0.5em}{}^1{S}_0$, $1s2s\hspace{0.5em}{}^3{S}_1$, and $1s2p\hspace{0.5em}{}^3{P}_0$ states of helium-like Xe${}^{52+}$, Au${}^{77+}$, and ${\mathrm{U}}^{90+}$ ions.

Figure 1

Angular correlation function (10) for the $1s2s\hspace{0.5em}{}^1{S}_0\to 1s^2\hspace{0.5em}{}^1{S}_0$ two-photon decay of helium-like xenon, gold, and uranium ions. Calculations obtained within the electric dipole 2E1 approximation (dashed line) are compared with those including all the allowed multipoles (solid line). Results are presented for the relative photon energies $y=0.1$ (upper panel) and 0.5 (lower panel).

Figure 2

Angular correlation function (10) for the $1s2s\hspace{0.5em}{}^3{S}_1\to 1s^2\hspace{0.5em}{}^1{S}_0$ two-photon decay of helium-like xenon, gold, and uranium ions. Calculations obtained within the electric dipole 2E1 approximation (dashed line) are compared with those including all of the allowed multipoles (solid line). Results are presented for the relative photon energies $y=0.1$ (upper panel) and 0.5 (lower panel).

Figure 3

Angular correlation function (10) for the $1s2p\hspace{0.5em}{}^3{P}_0\to 1s^2\hspace{0.5em}{}^1{S}_0$ two-photon decay of helium-like xenon, gold, and uranium ions. Calculations obtained within the dipole E1M1 approximation (dashed line) are compared with those including all of the allowed multipoles (solid line). Results are presented for the relative photon energies $y=0.1$ (upper panel) and 0.5 (lower panel).