Microwave Spectra of the Free Radicals OH and OD

Experiments are reported on the microwave spectra of the free OH radical. The radicals are produced by an electric discharge in concentrations near 10% at pressures of approximately 0.1 mm Hg. The spectra are detected by Zeeman modulation. They are due to direct transitions between the $\ensuremath{\Lambda}$-doublet levels of each rotational state in the ground vibrational level of the molecule. Spectra due to ${\mathrm{O}}^{16}$H, ${\mathrm{O}}^{18}$H, and ${\mathrm{O}}^{16}$D in $^{2}\ensuremath{\Pi}_{\frac{1}{2}}$ and $^{2}\ensuremath{\Pi}_{\frac{3}{2}}$ states have been observed in the 7.7 to 37 kMc/sec region. Intensity of the lines ranges from about 5\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}6}$ ${\mathrm{cm}}^{\ensuremath{-}1}$ to 5\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}8}$ ${\mathrm{cm}}^{\ensuremath{-}1}$.Van Vleck's theory of molecular energies in $^{2}\ensuremath{\Pi}$ and $^{2}\ensuremath{\Sigma}$ states is extended to include terms of order $\frac{{({E}_{\mathrm{rot}} or {E}_{\mathrm{fs}})}^{2}}{{{E}_{\mathrm{el}}}^{2}}$. The experimental results are in agreement with theoretical expectations to about one part in 2000 which is the order of accuracy of the theory. An improved agreement (to one part in 3500) is obtained if one allows a small variation (one part in 1400) of the electronic wave function from one rotational state to the next. The values of the molecular constants determined from the $\ensuremath{\Lambda}$-type doubling data are $4\ensuremath{\Sigma}\stackrel{}{\ensuremath{\Sigma}\ensuremath{-}\mathrm{s}\mathrm{t}\mathrm{a}\mathrm{t}\mathrm{e}\mathrm{s}}\frac{{(\ensuremath{-}1)}^{s}〈\ensuremath{\Pi}|A{L}_{y}+2B{L}_{y}|\ensuremath{\Sigma}〉〈\ensuremath{\Sigma}{|B{L}_{y}|}_{\ensuremath{\Pi}}〉}{({E}_{\ensuremath{\Sigma}}\ensuremath{-}{E}_{\ensuremath{\Pi}})}=\ensuremath{-}2361.37\ifmmode\pm\else\textpm\fi{}2.95 \mathrm{Mc}/sec\mathrm{in} {\mathrm{O}}^{16}\mathrm{H}\mathrm{and} \ensuremath{-}1548.99\ifmmode\pm\else\textpm\fi{}2.10 \mathrm{Mc}/sec\mathrm{in} {\mathrm{O}}^{16}\mathrm{D};$ $4\ensuremath{\Sigma}\stackrel{}{\ensuremath{\Sigma}\ensuremath{-}\mathrm{s}\mathrm{t}\mathrm{a}\mathrm{t}\mathrm{e}\mathrm{s}}\frac{{(\ensuremath{-}1)}^{s}{|〈\ensuremath{\Pi}|B{L}_{y}|\ensuremath{\Sigma}〉|}^{2}}{({E}_{\ensuremath{\Sigma}}\ensuremath{-}{E}_{\ensuremath{\Pi}})}=576.18\ifmmode\pm\else\textpm\fi{}1.64 \mathrm{Mc}/sec\mathrm{in} {\mathrm{O}}^{16}\mathrm{H} \mathrm{and} 161.94\ifmmode\pm\else\textpm\fi{}1.61 \mathrm{Mc}/sec\mathrm{in} {\mathrm{O}}^{16}\mathrm{D};$ $\frac{A}{B}=\ensuremath{-}7.444\ifmmode\pm\else\textpm\fi{}0.017$ in ${\mathrm{O}}^{16}$H and -13.954\ifmmode\pm\else\textpm\fi{}0.032 in ${\mathrm{O}}^{16}$D. The spectra include magnetic hyperfine structure from which the following values are obtained for parameters that describe the unpaired electron distribution in the molecule: ${(\frac{1}{{r}^{3}})}_{\mathrm{Av}}=(0.75\ifmmode\pm\else\textpm\fi{}0.25)\ifmmode\times\else\texttimes\fi{}{10}^{24} {\mathrm{cm}}^{\ensuremath{-}3}$ and ${(\frac{{sin}^{2}\ensuremath{\chi}}{{r}^{3}})}_{\mathrm{Av}}=(0.49\ifmmode\pm\else\textpm\fi{}0.01)\ifmmode\times\else\texttimes\fi{}{10}^{24} {\mathrm{cm}}^{\ensuremath{-}3}.$ The hyperfine structure, the molecular magnetic moment and the line intensities are strongly dependent on the extent of intermediate coupling in agreement with theoretical expectations. The microwave spectrum can be used in studying chemical properties of the radical. Its lifetime was determined to be near $\frac{1}{3}$ sec, and the effects of certain substances on radical concentration were examined.