An interference between electric quadrupole and Stark induced dipole amplitudes is observed for the first time. Measurement of this interference in atomic strontium permits the first experimental determination of the decay rate between the lowest ${}_{}{}^1{}_{}{}^{}\mathrm{P}$ and metastable ${}_{}{}^1{}_{}{}^{}\mathrm{D}$ levels in an alkaline-earth atom. We obtain the result A(5s5p-5s4d)=(3.85±0.94±0.5 3)×${10}^3$ ${\mathrm{s}}^{\mathrm{-}1}$.

• Received 23 December 1985

DOI:https://doi.org/10.1103/PhysRevLett.56.823