On the nature of the high-frequency relaxation in a molecular glass former:: A joint study of glycerol by field cycling NMR, dielectric spectroscopy, and light scattering

Fast field cycling H-1 NMR relaxometry is applied to determine the dispersion of spin-lattice relaxation time T-1(omega) of the glass former glycerol in broad temperature (75-360 K) and frequency (10 kHz-30 MHz) ranges. The relaxation data are analyzed in terms of a susceptibility chi(omega) proportional to omega/T-1 (omega), related to the second rank (l=2) molecular orientational correlation function. Broadband dielectric spectroscopic results suggest the validity of frequency temperature superposition above the glass transition temperature T-g. This allows to combine NMR data of different temperatures into a single master curve chi(omega tau(alpha)) that extends over 15 decades in reduced frequency omega tau(alpha), where tau(alpha) is the structural alpha-relaxation time. This master curve is compared with the corresponding ones from dielectric spectroscopy (l=1) and depolarized light scattering (l=2). At omega tau(alpha)<1, NMR susceptibility is significantly different from both the dielectric and light scattering results. At omega tau(alpha) > 1, there rather appears a difference between the susceptibilities of rank l=1 and l=2. Specifically, at omega tau(alpha)>>1, where the susceptibility is dominated by the so-called excess wing, the NMR and light scattering spectra (both l = 2) rather coincide with each other and are about three times more intense than the dielectric (l= 1) spectrum. This is explained by assuming that the high frequency dynamics correspond to only small-angle excursions. Below T-g, dielectric and NMR susceptibility compare well and exhibit an exponential temperature dependence. (C) 2008 American Institute of Physics.