Relativistic Two-Photon Emission: Lifetime of the21S0State of HeliumlikeKr34+

The two-photon ($2E1$) decay $2^1{S}_0\to 1^1{S}_0$ in the heliumlike ion ${\mathrm{Kr}}^{34+}$ has been observed and the lifetime measured. Departures are measured for the first time from the predictions of the nonrelativistic theory, and are sensitive to relativistic effects on the transition-matrix elements. The measured lifetime is $\tau \left(\mathrm{measured}\right)=34.08\mathrm{}\left(0.34\right)\mathrm{}\times {10}^{-12\mathrm{}}\mathrm{s}$ compared to $\tau \left(\mathrm{nonrelativistic}\right)=31.94\mathrm{}\left(0.01\right)\mathrm{}\times {10}^{-12\mathrm{}}\mathrm{s}$.