THE MOLECULAR-DYNAMICS AROUND THE GLASS-TRANSITION AND IN THE GLASSY STATE OF MOLECULAR ORGANIC-SYSTEMS - A H-2-NUCLEAR MAGNETIC-RESONANCE (NMR) STUDY

H-2-nuclear magnetic resonance (NMR)-spin-lattice relaxation experiments have been performed for studying the crossover from viscous (alpha-process) to secondary (beta-processes) dynamics in the van der Waals liquid orthoterphenyl and the H-bridged network glycerol. The essential and general features, observed in both systems, are the following: (a) a dominating alpha-process in the liquid and viscous regime; (b) a change from exponential to nonexponential spin-lattice relaxation as the temperature is lowered below a characteristic temperature above T(g); (c) the existence of a slow (> 10(-9) s) secondary reorientational process in the highly viscous regime; and (d) the existence of a fast (approximately 10)-12) s) local process in the glassy state. Whereas the slower process is shown to be the one known from dielectric studies, we attribute the fast mode to a beta-process found in quasielastic neutron scattering.