On the nature of slow beta-process in simple glass formers: A H-2 NMR study

We study the dynamics of the glass formers toluene and glycerol below the g lass transition temperature T-g by means of H-2 NMR techniques. Whereas for toluene a slow beta-process is observed in dielectric spectroscopy, this p rocess is not discernible in glycerol. We demonstrate that standard 1D and 2D NMR experiments cannot resolve the motional process because very small a ngular amplitudes are involved. However, applying the solid-echo technique and varying the interpulse delay, we detect changes of the spectra for tolu ene but not for glycerol, Performing simulations, we estimate molecular reo rientation of about 6 degrees. We conclude that the beta-process in simple organic glasses originates from highly restricted molecular reorientation o f essentially all molecules. No indications for "islands of mobility" are f ound.