Nonexponential nuclear Zeeman relaxation of tunnelling methyl groups

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Abstract
A model is proposed to explain the nonexponential nuclear Zeeman spin-lattice relaxation observed at low temperatures with some samples containing methyl groups. The theory predicts an observable nonexponential relaxation in the region 0.25 omega0< Deltat0, where h(cross) Deltatand h(cross) omega0are the tunnelling and Zeeman splittings respectively. Curves are presented which can be used to obtain Deltatfrom the experimentally observed weights and relaxation times of the fast and slowly relaxing components of the magnetization in the case of a very sharp tunnelling-splitting distribution. For Deltat<< omegaoor Deltat>> omegaothe present model agrees with the previous theories in predicting an exponential relaxation.