The gyromagnetic ratio of optically aligned $2{}_{}{}^3{}_{}{}^{}S_1^{}{}_{}{}^{}$ helium atoms has been measured by comparing its Zeeman resonance frequency with the proton transition frequency in the earth's magnetic field. The value of $\frac{{\gamma }_{\mathrm{He}}}{2\pi }$ extrapolated to zero light intensity is (2.80235 ± 0.000 03) ×${10}^6$ Hz ${\mathrm{G}}^{-1\mathrm{}}$. The use of unpolarized light to align the helium sample reduces the $g$ factor by an amount proportional to the pumping light intensity. This displacement results from an admixture of the excited-state ($2{}_{}{}^3{}_{}{}^{}P_2^{}{}_{}{}^{}$) $g$ factor into the $2{}_{}{}^3{}_{}{}^{}S_1^{}{}_{}{}^{}$ state during the optical pumping cycle. When the pumping light is circularly polarized, an additional shift appears, caused by virtual transitions related to the dispersion of the pumping light.

• Received 6 June 1968

DOI:https://doi.org/10.1103/PhysRev.175.36