We report measurements of the lifetimes of the $2^1$${\mathrm{S}}_0$ state of heliumlike ${\mathrm{Ni}}^{26+}$ and the $2^2$${\mathrm{S}}_{1/2}$ state of hydrogenlike ${\mathrm{Ni}}^{27+}$, both of which decay predominantly by two-photon emission. Our method differs from previous measurements of the lifetimes of similar states in that we require a coincidence between the two photons. Our lifetime of 217.1(1.8) ps for the hydrogenlike decay is in agreement with the theoretical value of 215.45 ps. The heliumlike decay lifetime of 156.1(1.6) ps is in fair agreement with the theoretical value of 154.3(0.5) ps.

• Received 16 January 1989

DOI:https://doi.org/10.1103/PhysRevLett.62.2809