Two-photon decay of the 21S0state in He-like bromine

We report a measurement of the lifetime of the 1s2s $^{1}$${\mathit{S}}_{0}$ state in two-electron ${\mathrm{Br}}^{33+}$. This state decays to the 1${\mathit{s}}^{2}$ $^{1}$${\mathit{S}}_{0}$ ground state only by the simultaneous emission of two photons. Ions in the 1s2s $^{1}$${\mathit{S}}_{0}$ level are produced by excitation of energetic bromine ions in a thin carbon foil. The lifetime is determined by measuring the change in the rate of two-photon coincidences as a function of the foil-detector separation. The lifetime measurement yields 39.32(32) ps in agreement with a theoretical value of 39.63(16) ps.