The Two-Photon Decay of 1s2s 1S0 States

In He-like systems the decay of the 1s2s S0 excited state to the 1s S0 ground state is not allowed. This excited state can only decay to the ground state via the emission of two photons. The spectral shape of the emitted continuum is determined by the complete structure of the atomic system as all bound and continuum P states contribute to the 2E1 decay. For very heavy atomic systems the P states ALSO have to be included and the normalized spectral shape changes with atomic number according to the relative strengthS of both, the electron-electron interaction and of the relativistic effects. A brief survey on the variation of the spectral shape of the two-photon continuum with atomic number is given and compared to experiments ranging from He-like Ni to He-like Au with special emphasis on the heavy relativistic system. The data compare well with fully relativistic calculations.