Evolution of the dynamic susceptibility of paradigmatic glass formers below the critical temperature Tc as revealed by light scattering

We have studied the molecular glass formers toluene and picoline in the liquid and glassy state by quasi-elastic light scattering applying tandem Fabry-Perot interferometry. Also, we perform an analysis of the susceptibility spectra of the inorganic glasses calcium potassium nitrate (CKN) and silica at T < T-g. At high temperatures the evolution of the susceptibility minimum in toluene and picoline is described by mode coupling theory (MCT), and the critical temperature T-c can be determined consistently. Below T-c the asymptotic laws of MCT fail, and the excess wing of the alpha process is rediscovered in a similar way as in the dielectric spectra. The wing contribution controls the evolution of the susceptibility minimum and is attributed to a secondary relaxation process different from the slow beta process. The fast dynamics spectra exhibit a trend to white noise at low frequencies (T < T-c). We conclude, regarding the fast dynamics, that MCT predictions hold also below T-c. In CKN the relaxation mechanism changes below T-g and the fast relaxation in both CKN and silica is reproduced by assuming thermally activated transitions in asymmetric double well potentials, while this is not possible for picoline or toluene. (C) 2002 Elsevier Science B.V. All rights reserved.