Two-photon decay in highly charged heavy ions: Spectral shape of the 2E1 (2 1S0 → 1 1S0) in He-like tin

We have measured the two-photon 2E1 transition from the singlet 1s2s 1S0 to the 1s21S0 ground state of helium-like tin. The experiment was performed by using initially Li-like tin ions in collision with a nitrogen gas target at the ESR storage ring at the energy of 300 MeV/u. The two-photon emission arises from the decay of the exclusively formed 1s2s states following the ionization of one K-shell electron from the Li-like ions. The obtained spectral shape of the 1s2s1S0 → 1s21S0 (2E1) transition was compared with the one calculated within a relativistic approach. The spectral shape is displayed and its sensitivity with regard to details in the theoretical description of the 2E1 decay is discussed.