Measurement of the Helium 23P0-23P1 Fine Structure Interval

We report the most accurate measurement of the helium fine structure splitting. The 23P0-23P1 energy splitting is 29616949.7±2.0kHz. Laser saturation spectroscopy, heterodyne pure frequency determination, and fluorescence detection were combined in a novel experimental approach. The absence of external perturbing magnetic fields, used in earlier experiments, lends confidence to our determined value and allows us to discriminate between contradictory results previously reported. This result, when combined with expected advances in theory, should yield a new value of the fine structure ?, which may help clarify a presently puzzling experimental situation.