Precision measurement of the 1s2s${\mathrm{}}^1$${\mathit{S}}_0$-1s2p${\mathrm{}}^1$${\mathit{P}}_1$ interval in heliumlike beryllium

Using fast-ion-beam laser spectroscopy, we have measured the 1s2s ${}_{}{}^1{}_{}{}^{}$ ${\mathit{S}}_0$–1s2p ${}_{}{}^1{}_{}{}^{}$ ${\mathit{P}}_1$ interval in Be i i i to be 16 276.774(9) ${\mathrm{cm}}^{\mathrm{-}1}$. Frequency calibration was provided by an iodine absorption cell and the iodine atlas of Gerstenkorn and Luc [Atlas du Spectre d’Absorption de la Mole$aa—cule d’Iode: Partie III (Editions du CNRS, Paris, 1978 )]. This work represents an improvement in precision over previous measurements by an order of magnitude and is in excellent agreement with a very recent calculation of Drake [Can. J. Phys. 66, 586 (1988)].