Tagged-particle motion in viscous glycerol: Diffusion-relaxation crossover

Incoherent neutron-backscattering and field gradient NMR are combined to measure the self-correlation function of protons in glycerol. Extensive simulations reveal and delimit systematic effects of multiple scattering. At microscopic wave numbers, tagged-particle motion couples strongly to structural relaxation. Between 1 and 0.1 A${\mathrm{̊}}^{\mathrm{-}1}$ a crossover to diffusional motion is observed: quasielastic stretching diminishes and the wave-number dependence of the mean relaxation time approaches ${\mathit{q}}^{\mathrm{-}2}$. © 1996 The American Physical Society.