Frequency Metrology of Helium around 1083 nm and Determination of the Nuclear Charge Radius

We measure the absolute frequency of seven out of the nine allowed transitions between the 2 3S and 2 3P hyperfine manifolds in a metastable 3He beam by using an optical frequency comb synthesizer-assisted spectrometer. The relative uncertainty of our measurements ranges from 1×10-11 to 5×10-12, which is, to our knowledge, the most precise result for any optical 3He transition to date. The resulting 2 3P–2 3S centroid frequency is 276 702 827 204.8(2.4) kHz. Comparing this value with the known result for the 4He centroid and performing ab initio QED calculations of the 4He-3He isotope shift, we extract the difference of the squared nuclear charge radii δr2 of 3He and 4He. Our result for δr2=1.074(3)  fm2 disagrees by about 4σ with the recent determination [ R. van Rooij et al. Science 333 196 (2011)].