NONEXPONENTIAL H-2 SPIN-LATTICE-RELAXATION AND SLOW TRANSLATIONAL EXCHANGE IN DISORDERED-SYSTEMS

We present an analysis of H-2 spin-lattice relaxation of deuterated be nzene in the glass former tricresyl phosphate. Whereas pronounced none xponential relaxation is observed well below the glass transition temp erature T-g (181 K), a crossover to exponential relaxation is found at T congruent to T-g. By assuming a certain distribution of barriers fo r the rotational jump process of benzene in the glassy matrix, we are able to describe all relaxation curves in the temperature range 20 K < T < 160 K. The crossover from nonexponential to exponential relaxatio n is described by additionally assuming translational exchange process es of benzene molecules between sites with different barriers for the rotational jumps. Within a simple model we are able to determine excha nge rates over more than two decades. Comparing the results with diffu sion data, we conclude that the obtained rates correspond to diffusion coefficients in the range 10(-18) < D/m(2) s(-1) < 10(-16) and that t ranslational rotational coupling does not hold near T-g. The further p otential of the method in order to determine very slow exchange proces ses is discussed.