First Pure Frequency Measurement of an Optical Transition in Helium: Lamb Shift of the23S1Metastable Level

The $2^3{S}_1-3^3{P}_0$ ${}_{}{}^4{}_{}{}^{}\mathrm{He}$ transition at ${\lambda }_0=389\mathrm{}$ nm is measured with respect to a previously frequency calibrated ${}_{}{}^{87}{}_{}{}^{}\mathrm{Rb}$ two-photon transition at $2{\lambda }_0=778\mathrm{}$ nm. The ${}_{}{}^4{}_{}{}^{}\mathrm{He}$ absolute frequency is 770 732 839 058 (190) kHz, with an accuracy of 2.4 parts in ${10}^{10}$. A Lamb shift value of +4057.61 (79) MHz, with 600 kHz uncertainty arising from the theoretical position of the $3^3{P}_0$ energy level, is extraced for the $2^3{S}_1$ level. This value is more than 2 orders of magnitude more accurate than the current best theoretical predictions.