Precision measurement of the 23S→33Ptransition inHe4

We report on a precision measurement of the 2 ${}_{}{}^3{}_{}{}^{}$S–3 ${}_{}{}^3{}_{}{}^{}$P transition in ${}_{}{}^4{}_{}{}^{}\mathrm{He}$. Doppler-free spectra of the transition were observed by exciting metastable helium atoms, produced by a chopped rf discharge, using 389-nm radiation generated by a frequency-doubled Ti-doped sapphire laser. A frequency measurement of the Doppler-free lines was made by interferrometric comparison of the Ti-doped sapphire frequency with that of an iodine-stabilized He-Ne laser using a 1-m evacuated Fabry-Pérot etalon. The measurement uncertainty was 2 parts in ${10}^9$ (3 standard deviations), which gives an improvement of a factor of 60 in the term value of the 3 ${}_{}{}^3{}_{}{}^{}$P level with respect to earlier measurements. This result makes it possible to determine the one electron Lamb-shift correction to the 3 ${}_{}{}^3{}_{}{}^{}$P state with an accuracy of 0.8%.