Precision laser spectroscopy of the $2^3S-2^3P$ transition of $^4$He

The fine-structure splitting of the $2^3P_J$ levels of $^4$He is of great interest for tests of quantum electrodynamics and for the determination of the fine structure constant $\alpha$. The $2^3S-2^3P$ transition absolute frequency, when combined with the point-like nucleus theoretical calculations, may provide accurate determination of the helium nuclear charge radius. Here we report our recent studies on the fine-structure splitting intervals, as well as the absolute frequency of the $2^3S-2^3P$ transitions. Laser spectroscopy was performed via $2^3P_J-2^3S_1$ transitions at 1083nm. The $2^3P_0-2^3P_2$ and $2^3P_1-2^3P_2$ intervals were determined to be 31 908 130.98(13) kHz and 2 291 177.56(19) kHz, respectively. Both intervals showed good agreements with the latest theoretical calculations. The absolute frequency of the $2^3S-2^3P$ centroid transition was measured with a relative accuracy of $5\times10^{-12}$.