RAMAN-SCATTERING MEASUREMENTS OF FRACTON IN SILICA AEROGELS

Silica aerogels are single phase, highly porous, silica based material s, obtained by hydrolysis of an alkoxysilane followed by polycondensat ion, gelation, and subsequent hypercritical drying. They can form frac tal structures at intermediate length scales L, smaller than the corre lation length xi and lager than a mean silica particle diameter a, xi > L > a. Vibrations of wave length lambda > xi are acoustic phonons an d those of wave length lambda < a are bulk and surface modes of silica particles. In the wave length region xi > lambda > a, new excitations have been predicted which are called ''fraction''. We succeed in obse rving fracton spectra scaling in the fractal range by means of Raman s cattering measurements.