We report quasielastic neutron scattering experiments exploring the $\alpha$ relaxation in polyisoprene over an unprecedented range in momentum transfer. Corroborating and validating earlier molecular dynamics simulations, the measurements reveal a crossover from a Gaussian regime of sublinear diffusion to a strongly non-Gaussian regime at short distances. We show that a consistent interpretation in terms of a distribution of finite jumps underlying the $\alpha$ process is possible. This model leads to a time-dependent non-Gaussian parameter exhibiting all features revealed so far from various simulations.

• Received 11 June 2002

DOI:https://doi.org/10.1103/PhysRevLett.89.245701