On the scaling in the rotational dynamics of molecular probes in salol and ortho-terphenyl: a possible role of the eneirgy landscape basins

The reorientational dynamics of a stiff paramagnetic tracer dissolved in the glass former salol is investigated by means of electron spin resonance over a broad temperature range. The Debye-Stokes-Einstein law describing rotational diffusion in simple liquids, is found to break down in the supercooled region where the diffusion is less temperature dependent than the viscosity. Over a large temperature interval a simple power law relates diffusion and viscosity, whereas at lower temperatures the decoupling is stronger and an activated dynamics is observed. These experiments are discussed together with previous data concerning other tracer/glass former couples. Starting from some observed common features, an attempt is made to obtain a unifying interpretation of the data in the framework of the energy landscape picture.