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Nuclear spin relaxation in methane gas Beckmann, Peter Adrian

Abstract

Nuclear spin relaxation in low density methane gas (CH₄) is investigated both experimentally and theoretically. The 30 MHz nuclear magnetic resonance experiments were performed at temperatures of 77K, 110K, 150K and 295K over a density range of 0.0046 to 0.040, 0.008 to 2.0, 0.006 to 6.0 and 0.006 to 17 amagat, respectively. The recovery of the observed magnetization could be characterized by a single relaxation rate to within experimental error. The manner in which the relaxation rate is extracted from the data, along with a statistical and (known) systematic error is discussed in detail. A theory is developed in which the Fermi-Dirac statistics of the nuclear spins (protons) and the centrifugal distortion of the molecule play a central role. The calculations imply that, in general, the relaxation should be non-exponential. At nuclear Larmor frequencies coincident with molecular centrifugal distortion frequencies, an observable departure from exponential relaxation is predicted by the calculations. On the other hand, at 30 MHz, which is well away from centrifugal distortion frequencies, the departure from exponential relaxation is predicted to be too small to be observed. This is consistent with the observed single decay constant. In addition, the magnitudes of the calculated longitudinal relaxation rates are shown to be consistent with experiment. Finally, new experiments are suggested, based on the theory developed, that should unequivocally show the importance of the centrifugal distortion splittings.

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