Hyperfine structure of the 33 P state of3He and isotope shift for the 23 S-33 P 0 transition

The hyperfine structure of the³He 1s 3p³P state and the³He-⁴He isotope shift is determined by high precision measurements of the 1s2s³S₁-1s 3p³p³P_J transition frequencies near 389 nm. A direct frequency measurement is made without the need for wavelength calibration by tuning a single laser to the atomic frequency, and using a novel heterodyne method to observe beat frequencies with a stable reference laser. A fit to a theoretical model of hyperfine structure is used to determine the hyperfine shifts. Additional off-diagonal mixing effects are investigated to resolve a possible systematic discrepancy in the hyperfine intervals. The final isotope shift without hyperfine structure of 42184308±165 kHz is used to deduce an rms nuclear charge radius for³He of 1.956±0.042 fm. This is in good agreement with other values obtained from atomic isotope shift measurements, and a recent theoretical value of 1.958±0.006 fm. The present result helps to resolve substantial differences in the³He nuclear radius derived from electron-nuclear scattering measurements, and it provides a significant test of the nuclear three-body problem.