We have not to be surprised to find time correlations containing Bessel functions. For instance, in Hall and Helfand, 1982, we can find them in time-correlation functions of the conformational state relaxation in polymers. Hall and Helfand studied the "relaxation processes in polymer molecules which proceed via conformational transitions of the chain backbone from one rotational isomeric state to another. … The resulting correlation functions contain a modified Bessel function which is associated with the diffusional nature of the process. This functional form has recently proven useful in fitting time-correlation functions determined in polymer simulations, which indicates that it will be of value in fitting data obtained in relaxation experiments on polymers" .