SPIN RELAXATION DRIVEN BY DYNAMIC DISORDER

A dynamic disorder model is developed to describe the random changes o f a local structure due to the environment rearrangements. The picture incorporates in a simple way both ''local'' and ''cooperative'' featu res of the structural relaxation. If applied to the spin relaxation of a two-level system, the model accounts for the crossover between the homogeneous and the heterogeneous relaxation which is usually found in disordered systems. A good agreement with recent NMR results on the r otational dynamics in supercooled fluids is found.