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 an d molecular-dynamic simulations in amorphous silica are compared to obtain the Raman coupling coefficient C(omega) and, in particular, its low-frequen cy limit. This study indicates that in the omega --> 0 limit C(omega) extra polates to a nonvanishing value, giving important indications on the charac teristics of the vibrational modes in disordered materials; in particular o ur results indicate that even in the limit of very long wavelength the loca l disorder implies nonregular local atomic displacements.