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 ( ω ) 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 χ ″ ( ω ) ∝ ω ∕ T 1 ( ω ) , 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 χ ″ ( ω τ α ) that extends over 15 decades in reduced frequency ω τ α , where τ α is the structural α -relaxation time. This master curve is compared with the corresponding ones from dielectric spectroscopy ( l = 1 ) and depolarized light scattering ( l = 2 ) . At ω τ α < 1 , NMR susceptibility is significantly different from both the dielectric and light scattering results. At ω τ α > 1 , there rather appears a difference between the susceptibilities of rank l = 1 and l = 2 . Specifically, at ω τ α ⪢ 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.