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Nuclear spin-lattice relaxation in solid methane at low temperatures. De Wit, Gerald Aloysius

Abstract

The spin-lattice relaxation time T₁ has been measured in the temperature range 1.2 to 55°K at 28.5 mcs. for the proton resonance, and at 4.4 mcs for the deuteron resonance using N.M.R. pulse techniques. The proton T₁ has been measured for CH₄, CH₃D, CD₃H, 50%CH₄-50%Kr, 90%CH₄-10%Kr, 67%CD₄-33%CH₄, 10%CD₄-90%CH₄, and also for CH₄ at 4.4 mcs. The deuteron T₁ has been measured for CD₄, CD₃H, and 67%CD₄-33%CH₄. It is found that a drastic change in the temperature dependence of T₁ occurs in the temperature region below the phase transitions and that at most of the phase transition temperatures there is either a discontinuous change in T₁ or a change in the slope of T₁ versus T. A minimum in T₁ is found at low temperatures for all the systems studied. An analysis of the data based on conventional N.M.R. theory shows in most cases that the correlation time Ƭc α T¯⁷ in the neighbourhood of 20°K, and that Ƭc is almost independent of temperature near 1.2°K. It is postulated that phonon-molecular interactions, involving direct and Raman processes, can account for the temperature dependence of Ƭc. The values of T₁ at the minimum are completely determined by conventional theory. In most cases, however, the predicted values are of the order of 20 times too short. An unexplained minimum in T₁ was observed in CH₄, CH₄-CD₄, and CH₄-Kr mixtures above the upper phase transitions. To investigate the origin of some of the inadequacies of the conventional theory, the two energy level scheme proposed by Colwell, Gill, and Morrison (1965) is used, where each of the two levels may be degenerate. Simple rate equations are used to calculate the conditional probabilities and the correlation functions for the two level model. It is found that the effective interaction strength is temperature dependent, that the correlation function can be described by a simple exponential under certain conditions, and that the interaction strength has no simple relationship with the classical value.

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