title: 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 2 (3)S and 2 (3)P hyperfine manifolds in a metastable (3)He 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 ^{3}He transition to date. The resulting 2 (3)P-2 (3)S centroid frequency is 276,702,827,204.8(2.4) kHz. Comparing this value with the known result for the (4)He centroid and performing ab initio QED calculations of the (4)He-(3)He isotope shift, we extract the difference of the squared nuclear charge radii δr(2) of (3)He and (4)He. Our result for δr(2)=1.074(3) fm(2) disagrees by about 4σ with the recent determination [R. van Rooij et al., Science 333, 196 (2011)].
authors.affiliation: Istituto Nazionale di Ottica-CNR (INO-CNR), Via Nello Carrara 1, I-50019 Sesto Fiorentino, Italy. pablo.canciopastor@ino.it
authors.forename: P
authors.initials: P
authors.lastname: Cancio Pastor
authors.forename: L
authors.initials: L
authors.lastname: Consolino
authors.forename: G
authors.initials: G
authors.lastname: Giusfredi
authors.forename: P
authors.initials: P
authors.lastname: De Natale
authors.forename: M
authors.initials: M
authors.lastname: Inguscio
authors.forename: V A
authors.initials: VA
authors.lastname: Yerokhin
authors.forename: K
authors.initials: K
authors.lastname: Pachucki