Spectral shape of the two-photon decay of the 2 1S0 state in he-like tin.

The spectral distribution of the $1s2s\text{ }^{1}S_{0}\ensuremath{\rightarrow}1{s}^{2}\text{ }^{1}S_{0}$ two-photon decay of He-like tin was measured using a novel approach at the gas-jet target of the ESR storage ring. Relativistic collisions of Li-like projectiles with low-density gaseous matter have been exploited to selectively populate the desired $1s2s$ state. Compared to conventional techniques, this approach results in a substantial gain in statistical and systematic accuracy, which allowed us to achieve for the first time a sensitivity to relativistic effects on the two-photon decay spectral shape as well as to discriminate the measured spectrum for Sn from theoretical shapes for different elements along the He-isoelectronic sequence.