Thermal diffusivity and structural relaxation in a supercooled liquid by a holographic grating technique

The decay of a spatial transient temperature grating created within the molecular glass former phenolphthalein-dimethylether (PDE) relates to the hydrodynamic heat diffusion and the dynamics of local density fluctuations. The characteristics of the latter process agree well with the structural α relaxation observed by photon correlation and dielectric spectroscopy near and above the glass transition temperature Tg. When the α-relaxation time approaches the experimental timescale set-in by the thermal diffusivity the specific heat undergoes a dispersion. In either side of this dynamic regime, heat diffusion and α relaxation are independent modes. ‘‘Two fluid’’ models of the vitreous state conform to the experimental observations.