Dynamics of glass-forming liquids.: IV.: True activated behavior above 2 GHz in the dielectric α-relaxation of organic liquids

We have measured the dielectric relaxation of butylbenzene of the glass-former propylbenzene in the frequency range 10(-2) Hz to 2 X 10(10) Hz in order to characterize the variation of relaxation times with temperature for these low loss liquids. Additionally, salol has been remeasured above 1 GHz with improved resolution. Using the sensitive data representation [-d log(10)(f(max)Hz)/d(1/T)](-1/2) vs 1/T we find demarcation temperatures TA, at which the temperature dependence changes from a Vogel-Fulcher type law within the limits T-B less than or equal to T less than or equal to T-A to Arrhenius behavior for T>T-A, corresponding to a position of the loss peak f(max)>2 GHz. The activation energies derived from dielectric relaxation data for T>T-A are associated with the energy of vaporization, D eta proportional to Delta E-vap. A comparison of dielectric relaxation times tau(D) to viscosity data in this wide range of temperatures suggests the relation tau(D) proportional to eta/T rather than tau(D) proportional to eta. (C) 1998 American Institute of Physics. [S0021-9606(98)50415-1].