Measurement of the Helium23P0−23P1Fine Structure Interval

We report the most accurate measurement of the helium fine structure splitting. The $2^{3}P_{0}\ensuremath{-}2^{3}P_{1}$ energy splitting is $29616949.7\ifmmode\pm\else\textpm\fi{}2.0\mathrm{kHz}$. 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 $\ensuremath{\alpha}$, which may help clarify a presently puzzling experimental situation.