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 A-doublet levels of each rotational state in the ground vibrational level of the molecule. Spectra due to 0 16 H, 0 18 H, and 0 16 D in II* and II* states have been observed in the 7.7 to 37 kMc/sec region. Intensity of the lines ranges from about 5X10~~6 cm"" 1 to 5X10 -8 cm"" 1 .Van Vleck's theory of molecular energies in 2 n and 2 S states is extended to include terms of order (E vo t or Ef s ) 2 /E e i 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 A-type doubling data are 4 2 (-l)*(n|4i y +2BZ y |S>(S|5L I ,|n>/(fe-£n) 2-states = -2361.37±2.95 Mc/sec in 0 16 H and -1548.99±2.10 Mc/sec in 0 16 D; 4 S (-l) s |(n|5L 2/ |S>| 2 /(^s-^n) = 576.18±1.64 Mc/sec S-states in 0 16 H and 161.94zbl.61 Mc/sec in 0 16 D; A/B = -7.444±0.017 in 0 16 H and -13.954±0.032 in O l6 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: and (1A 3 )AV= (0.75±0.25)X10 24 cm"* (sin 2 xA 3 )Av= (0.49±0.01) X10 24 cm~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 J sec, and the effects of certain substances on radical concentration were examined.

More than 90% of the photons emitted from forward-biased GaAs diodes have energies hv higher than the applied voltage V. Thus, a portion of the energy of these photons must come from lattice heat. This portion is 3% of hv for photons at the peak of the incoherent emission spectrum. An upper limit of (hv-qV) is estimated from thermodynamics. This difference is larger at 78 than 27°K, in agreement with theory. Also, the dependence of the effect on voltage and current is in fair agreement with expectations. At high forward currents, near the threshold for stimulated emission, hv is about equal to qV. The removal of heat, in the form of photon energy from the crystal, should in principle lead to refrigeration. The main requirement for net cooling is a quantum efficiency (photons/electron) higher than 0.97. At 10 mA, with an assumed quantum efficiency of 0.99, the heat removal rate (per diode) is estimated as 3X10 -4 W compared to a Joule heating rateof 5X10-5 W.

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Microwave spectroscopic measurements on the partially deuterated forms of CH3Cl, CH3Br, and CH3I allow a determination of the structural parameters which is less subject to zero-point vibration errors than other methods. In addition, examination of all measured isotopic species reveals that the C—D distance is shorter than C–H by 0.009A, and the D—C—D angle greater than that for H–C–H by about 12 minutes. Best values of structural parameters for the species containing the light isotope of hydrogen only are C–Cl distance 1.781, C–Br 1.939, C–I 2.139, C–H 1.11 (same for all halides), H–C–H angle 110°31′ for CH3Cl, 111°14′ for CH3Br, and 111°25′ for CH3I. Deuteration not only gives a change in the C–H bond but also a well-correlated change in the quadrupole coupling constant of the halogen.

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