Microwave Zeeman Effect of Free Hydroxyl Radicals

Abstract

Paramagnetic resonance absorption at 3-cm wavelength has been observed in the products of an electric discharge in low-pressure ${\mathrm{H}}_2$O and ${\mathrm{D}}_2$O vapor. The spectra are of the electric dipole type, and arise from $\Lambda$-type doubling transitions in low-lying rotational levels of the free ${\mathrm{O}}^{16}$H and ${\mathrm{O}}^{16}$D radicals. The theory of the Zeeman effect in ${}_{}{}^2{}_{}{}^{}\mathrm{II}$ levels of light diatomic radicals is extended to the general intermediate coupling case, and is used for a detailed analysis of the observed spectra. Numerical results of this analysis include molecular $g$ factors precise to within 3 parts in ${10}^5$, and magnitudes of the $\Lambda$-type doubling intervals in several low rotational levels. The measured $g$ factors are compared with theory, including small corrections for molecular rotation, the anomalous spin magnetic moment of the electron, and estimated relativistic effects. This comparison yields the value 0.67±0.01 for the molecular matrix element ($\Pi \left|L_y\right|\Sigma$), and also brings to light serious discrepancies between the present experimental results and earlier measurements of the $\Lambda$-type doubling in OH and OD. The paramagnetic resonance spectra also exhibit hyperfine structure, from which are derived molecular constants that describe the distribution of unpaired electrons about the H or D nucleus.