Microwave Zeeman Effect of Free Hydroxyl Radicals:Π122Levels

Two new paramagnetic resonance spectra of the free ${\mathrm{O}}^{16}$H radical have been observed at 3 cm wavelength in the products of an electric discharge in water vapor. The spectra arise from $\Lambda$-type doubling transitions in the ${}_{}{}^2{}_{}{}^{}\Pi _{\frac{1}{2}}^{}{}_{}{}^{}$, $J=\frac{3}{2}$ and ${}_{}{}^2{}_{}{}^{}\Pi _{\frac{1}{2}}^{}{}_{}{}^{}$, $J=\frac{5}{2}$ levels. Molecular magnetic moments and hyperfine structure constants measured from the spectra lead to refinements in the theory developed earlier to account for similar measurements on ${}_{}{}^2{}_{}{}^{}\Pi _{\frac{3}{2}}^{}{}_{}{}^{}$ levels. A consistent analysis of the combined ${}_{}{}^2{}_{}{}^{}\Pi _{\frac{1}{2}}^{}{}_{}{}^{}$ and ${}_{}{}^2{}_{}{}^{}\Pi _{\frac{3}{2}}^{}{}_{}{}^{}$ data is then possible, and yields the following major results. From the measured magnetic moments we find $g_s\left(\mathrm{OH}\right)=g_s\left(\mathrm{free}\mathrm{electron}\right)\pm 0.0002$, $\lambda =-7.504\mathrm{}\pm 0.003$, where $g_s\left(\mathrm{OH}\right)$ is the spin $g$ factor of a molecular electron and $\lambda$ is the ratio of the spin-orbit coupling constant to the rotational constant of the ground state. This value of $\lambda$ disagrees with an earlier microwave measurement. Values of matrix elements connecting the ground and first excited electronic states are also derived from the measured magnetic moments. Analysis of the hyperfine structure yields the molecular constants (in units of ${10}^{24}$ ${\mathrm{cm}}^{-3\mathrm{}}$): ${〈\frac{1}{r^3}〉}_{\pi }=(1.089\mathrm{}\pm 0.008)$, ${\Psi }^2\mathrm{}\left(0\right)=(0.113\mathrm{}\pm 0.001)$, $〈\frac{(3\mathrm{}{cos}^2\chi -1)}{r^3}〉=(1.125\mathrm{}\pm 0.008)$, and