Nonexponential nuclear Zeeman relaxation of tunnelling methyl groups

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 omega 0 < Delta t <6 omega 0 , where h(cross) Delta t and h(cross) omega 0 are the tunnelling and Zeeman splittings respectively. Curves are presented which can be used to obtain Delta t from 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 Delta t << omega o or Delta t >> omega o the present model agrees with the previous theories in predicting an exponential relaxation.