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Paramagnetic resonance absorption at 3-cm wavelength has been observed in the products of an electric discharge in low-pressure H 2 0 and D 2 0 vapor. The spectra are of the electric dipole type, and arise from A-type doubling transitions in low-lying rotational levels of the free 0 16 H and 0 16 D radicals. The theory of the Zeeman effect in 2 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 A-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=b0.01 for the molecular matrix element (n|Zj2), and also brings to light serious discrepancies between the present experimental results and earlier measurements of the A-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. 4 M. Tinkham and M. W. P. Strandberg, Phys. Rev. 97, 951 (1955). * J. M. Hendrie and P. Kusch, Phys. Rev. 107, 716 (1957). 114 MICROWAVE ZEEMAN EFFECT OF FREE HYDROXYL RADICALS 115 2 n3/2, J-=5/2 (mj) + -(mr-lF V2 Zl\Z ~Zl

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The microwave spectrum of the K -doublet transitions of the free radical HCO has been observed in the N = 1, 2, and 3 rotational levels. The spectrum was analyzed and the following spectroscopic parameters were determined from the data: B − C = 2836.38 ± 0.45 MHz, ε = 224.62 ± 0.65 MHz, |εz z| = 8.12 ± 0.48 GHz, τ = 5.06 ± 0.13 MHz, D = 0.049 ± 0.019 MHz. The newly measured rotational constant was used along with previously measured constants to determine that the geometry of the HCO radical is rCH = 1.11 Å, rCO = 1.17 Å, θHCO = 127°.