Two-photon energy distribution from the decay of the 2 ^{1}S_{0} state in He-like uranium

We have performed a measurement of the spectral shape from the two-photon decay of the 1s2s^{1}S_{0} state in He-like uranium. The two-photon emission followed the ionization of initially Li-like uranium ions in collisions with a N_{2} gas-jet target. The measured shape of the two-photon energy distribution shows good agreement with results of the relativistic calculations that take into account the electron-electron interaction rigorously up to the first order in quantum electrodynamic perturbation expansion. From the full width at half maximum of the measured two-photon energy distribution, we confirm the theoretically predicted modification of the shape due to the relativistic effects.