Brillouin scattering investigation of fast dynamics in glass forming systems

We review the results of recent Brillouin light and X-ray scattering investigations of the dynamic structure factor of three fragile glass-forming systems: ortoterphenyl, polybutadiene, and diglycidyl ether of bisphenol-A. In these systems, evidence is given of three processes responsible for the acoustic loss at high frequencies: the alpha relaxation, a secondary relaxation of intramolecular nature and an 'instantaneous' process. New data for the intermediate glass former glycerol are also presented and compared with literature ones, suggesting the absence of the intramolecular relaxation in this system.