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Magnetic relaxation times of hydrogen and fluorine in anhydrous hydrofluoric acid cannot be accounted for on the assumption of a pure dipole-dipole interaction between the hydrogen and the fluorine nuclei in the same molecule. However, the introduction, in the Hamiltonian, of a scalar term AI·S resulting from the indirect electron-coupled interaction between the two nuclei removes all the discrepancies between the calculated and observed decay times. Although the splitting due to this scalar term is smeared out by the rapid chemical exchange of the protons with the traces of water present in the acid, the nuclear Overhauser effect provides the extra parameter required to separate the dipole-dipole interaction from the scalar interaction and to calculate separately the scalar splitting and the exchange rate of the protons. The value obtained for the splitting is A/h=615 cps. The same method has been applied to investigate the structure of the HF molecule in solutions. The experimental results can be explained qualitatively by the following picture: (a) The fluorine nuclei form long chains that move more slowly than the Larmor frequency of fluorine. (b) The chemical exchange with the relatively large quantity of water present makes the motion of the hydrogen nuclei fast with respect to the hydrogen Larmor frequency. The results are: Long relaxation times for hydrogen with T1 and T2 equal, relatively short relaxation times for fluorine resonance, with T2 shorter than T1. Solutions of salts of hydrofluoric acid (KF) show the same character as solutions of the acid.

Previous theories to explain the variation of photoconductivity upon saturation of electron spin resonance, as observed by Lépine in silicon, predict an effect 10 to 100 times smaller than experiment. In the present model we show that, due to the shorter lifetime of electron-hole pairs in singlet configuration, the steady state spin distribution shows a surplus of triplet pairs. Saturation of resonance restores the random distribution, resulting in a shortening of the recombination time. The relative variation can be as large as 10 %, and is field independent as confirmed by experiment

Torrey 1 has observed the free precession of nuclear spins around an rf field H lf fixed in a frame rotating at the Larmor frequency a) 0 =yH 0 around a large dc magnetic field H 0 . He showed that, for an H 1 much larger than the inhomogeneity of H 0 , the latter has a negligible effect on the decay of the spin magnetization which is mainly due to the inhomogeneity of H x . We report here on a method of overcoming the inhomogeneity of H x by the production of echoes in the rotating frame ("rotary echoes") which are very similar to the usual spin echoes. 2 The rotary echoes, however, have some additional specific features that make them particularly suitable for the measurement of long relaxation times.Consider the rotating frame with the z -axis along H 0 and the x -axis along the rf field H x . If at the time £ = 0 we suppose that the spin magnetization M is along the z -axis, M will precess in the yz plane and, at t = r, the angle of precession is

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