Absolute Frequency Measurements of the23S1→23P0,1,2Atomic Helium Transitions around 1083 nm

We measure the frequency of the $2^3{S}_1\to 2^3{P}_{0,1,2}$ transitions of helium in a metastable beam using an optical frequency comb synthesizer. The relative uncertainty of these measurements ranging from $5\times {10}^{-11}$ to $7\times {10}^{-12}$ is, to our knowledge, the most precise result for any optical helium transition. Considering existing accurate values of the $2^{3}P$ fine structure, we measure a centroid value of the $2^{3}S-2^{3}P$ frequency of 276 736 495 624.6(2.4) kHz, improving the previous interferometric measurement by 30 times. New accurate values of the $2^{3}S-2^{3}P$ and $2^{3}P$ Lamb-shift energies are obtained.