On the nature of slow beta-relaxation in supercooled liquids

We propose a model for reorientational motions of molecules associated with secondary beta-relaxation in supercooled liquids. The secondary relaxation is attributed to relaxation within a given local minimum, while the primar y relaxation is attributed to transitions between distinct free-energy mini ma. We find that (i) at the temperature where the peak frequency of the ext rapolated beta-relaxation intersects the alpha-relaxation, the actual and t he extrapolated spectra differ in their time constants by approximately one decade; (ii) there is no clear division between the imaginary part of the dielectric susceptibility for the alpha- and the beta-relaxation for temper atures larger than 1.1 T-g. Thus, one must proceed with caution to extrapol ate low temperature data of beta-relaxation to higher temperatures in order to estimate the temperature at which the time scales for the two processes cross. The relaxation times for the alpha- and the beta-processes cannot c ross except at high temperature, where only the primary relaxation remains.