Energy landscape in molecular glasses probed by high-resolution dielectric experiments

Dielectric measurements between the glass transition temperature T(g) and about 3 K are reported for a supercooled plastic crystal and several organic molecular glasses that exhibit a secondary loss peak (beta process) close to T(g). Above about 10 K and up to T(g)/2, the dynamics can be described as a thermally activated process taking place within an energy landscape characterized by a bimodal distribution of barrier heights. This energy landscape is revealed by subjecting the dielectric loss data to an appropriate scaling involving a single free parameter. At temperatures above about T(g)/2, the scaling deteriorates for most glasses due to the emergence of the so-called excess wing.