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 $1s2s{\\phantom{\\rule{0.16em}{0ex}}}^{1}{S}_{0}$ 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.