The Raman coupling function in amorphous silica and the nature of the long-wavelength excitations in disordered systems

New Raman and incoherent neutron scattering data at various temperatures and molecular-dynamic simulations in amorphous silica are compared to obtain the Raman coupling coefficient C(omega) and, in particular, its low-frequency limit. This study indicates that in the omega --> 0 limit C(omega) extrapolates to a non vanishing value, giving important indications on the characteristics of the vibrational modes in disordered materials; in particular our results indicate that even in the limit of very long wavelength the local disorder implies non regular local atomic displacements.