One of the main open fields in the theory of topologically disordered solids regards the understanding of the mechanisms involved in the attenuation of acoustic excitations [1]. Despite the great amount of experimental and theoretical investigations of the subject, it appears that a complete and coherent description of the topic is far from being reached.
Experiments and numerical simulations on vitreous SiO_2 and glycerol [2,3] show that the sound attenuation Gamma varies quadratically as a function of the exchanged momentum  Q and that it is temperature independent in the high-Q region,  Q ~ 1-10 nm^(-1). On the contrary it is well known that at low Q values, up to the region investigated by Brillouin light scattering (BLS), the sound attenuation shows a marked temperature dependence [4].
We report measurements of the width of the Brillouin peak on vitreous SiO_2 for Q values in the region between BLS and x-rays scattering. The experiment has been carried out at the new Inelastic Ultra Violet Scattering (IUVS) beam line at the Elettra Shyncrotron radiation facility in Trieste. The beam line operates with incident light of wavelength tunable in the range 260-110~nm.
We have measured the dynamic structure factor at temperatures between 20 and 600~K, for  Q values in the range  0.07 - 0.14  nm^(-1). The experiment strongly suggests that the sound attenuation mechanism goes through a change of regime in this range of exchanged momenta. This change of regime corresponds to a crossover between the low frequency region dominated by dynamical, temperature activated, mechanisms and the high frequency static behavior.
Besides the experimental measures, we are performing some numerical simulations on simple models [5] with the aim of better understanding the role of disorder in the mechanism of sound attenuation.
 
 
 

[1] A. Fontana, G. Viliani (guest editors), Philos. Mag. B 1 (2002)  Special issue: Eight International Workshop on Disordered Systems, Andalo, 2001
[2] G. Ruocco et al.,  Phys. Rev. Lett.  83, 5583 (1999)
[3] B. Ruzicka et al.,  Phys. Rev. B   69, 100201(R) (2004)
[4] R. Vacher et al.,  Non-Crystalline Solids  45, 397 (1981)
[5] L. Angelani et al.,  Phys. Rev. Lett .  84, 4874 (2000)