Measurement ofgFactors by Quantum Beats in the OH Free Radical

Using pulsed optical excitation we have measured the $g$ factors of twelve hyperfine levels ($K=1\mathrm{} \mathrm{through} 6$) of the $A{{}_{}{}^2{}_{}{}^{}\Sigma _{\frac{1}{2}}^{}{}_{}{}^{}}^{+}$ state of OH by observation of quantum beats. The results are consistent with Hund's case (b) coupling to within the experimental uncertainty of 0.35% (principally limited by magnetic field calibration). However, the ratios of the $g$ factors of a given hyperfine doublet, which are independent of field and accurate to about 0.15%, show significant discrepancies from theory.