Frequency metrology of helium around 1083 nm and determination of the nuclear charge radius
Abstract
We measure the absolute frequency of seven out of the nine allowed transitions between the 23 S and 23 P 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 23 P-23 S 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 δ r2 of 3He and 4He. Our result for δ r2=1.074 (3) fm2 disagrees by about 4\,σ with the recent determination [R. van Rooij et al., Science 333, 196 (2011)].
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