Brillouin scattering investigations of fast dynamics in glass forming systems L. Comez a , D. Fioretto a, * , G. Monaco b , G. Ruocco c a Dipartimento di Fisica and INFM, Universit  a di Perugia, I-06123, Perugia, Italy b European Synchrotron Radiation Facility, B.P. 220 F-38043, Grenoble cedex, France c Dipartimento di Fisica and INFM, Universit  a di Roma ‘La Sapienza’, I-00185 Rome, Italy Abstract We review the results of recent Brillouin light and X-ray scattering investigations of the dynamic structure factor of threefragileglass-formingsystems:ortoterphenyl,polybutadiene,anddiglycidyletherofbisphenol-A.Inthesesystems, evidence is given of three processes responsible for the acoustic loss at high frequencies: the a relaxation, a secondary relaxationofintramolecularnatureandan‘instantaneous’process.Newdatafortheintermediateglassformerglycerol are also presented and compared with literature ones, suggesting the absence of the intramolecular relaxation in this system. Ó 2002 Elsevier Science B.V. All rights reserved. PACS: 78.35.+c; 61.20.LC; 78.70.Ck; 83.50.Fc 1. Introduction The dynamical processes that lead to the su- percooling of a liquid have been successfully in- terpretedbythemodecouplingtheory(MCT)ina number of real and simulated systems [1]. The MCT describes the slowing down of the structural dynamics of a liquid in terms of the development of non-linear coupling between density fluctua- tions, that, in the idealized version, is responsible of the structural arrest at a critical temperature T c . The MCT has motivated an extensive experimen- talandnumericalwork,whichhastypicallyshown a remarkable agreement between the predictions of the theory and experimental results on the dy- namics of supercooled systems probed by different techniques in different momentum and frequency ranges. Some recent work is collected in this same volume,andsomeearlierworkcanbefoundinthe proceedings of previous meetings [2]. Anoticeableexceptionisgivenbythedynamics of density fluctuations in the hydrodynamic re- gime, typically studied by Brillouin light scatter- ing. In fact, to our knowledge, the only positive test of MCT by Brillouin light scattering has been reported in CKN, where the density fluctuation spectra were fitted using a memory function di- rectly obtained from the depolarized light spectra measuredinthesamesystem[3].Forothersystems theprocedureadoptedforCKNwasnotsuccessful Journal of Non-Crystalline Solids 307–310 (2002) 148–153 www.elsevier.com/locate/jnoncrysol * Corresponding author. Tel.: +39-075 575 3067. E-mail address: fioretto@pg.infn.it (D. Fioretto). 0022-3093/02/$ - see front matter Ó 2002 Elsevier Science B.V. All rights reserved. PII:S0022-3093(02)01453-9