High-frequency dielectric spectroscopy of glass-forming liquids

We give an overview of our recent results from broadband dielectric spectroscopy on various glass forming liquids as glycerol, [Ca(NO3)(2)](0.4)[KNO3](0.6), [Ca(NO3)(2)](0.4)[RbNO3](0.6), propylene-carbonate, and Salol. For the first time the dielectric loss has been investigated systematically in the crossover regime from the alpha-relaxation to the far-infrared (FIR) response. For all materials investigated we observe a minimum in epsilon ''(nu) in the GHz-THz range with a significantly sublinear high-frequency wing. This behavior cannot be explained by a simple transition from the alpha-relaxation peak to the FIR bands but has to be attributed to additional fast precesses. We compare our results to the predictions of the mode coupling theory of the glass transition.