Depolarized Raman scattering from disordered systems

Until now the low-frequency light scattering spectrum in fluids of isotropic molecules or atoms has been referred to as a collision-induced process reflecting somehow the distribution of velocities of the colliding particles (CILS). In this article a rather different interpretation is presented, suggested by a comparative analysis of Rayleigh spectra in amorphous solids and liquids, which seems to be more suitable than the binary-collision approach in describing the spectra of dense fluids. Two points are of basic importance: 1) the existence of phononlike excitations in dense fluids at high wave vectors, observed by neutron scattering and simulated by molecular dynamics; 2) a careful analysis of the light scattering in disordered media, which shows that a disorder-induced incoherent contribution is present in such systems. This reflects the translational excitation spectrum of the system or its vibrational density of states. These notes are essentially concerned with this second point, so that we will be speaking essentially of disordered solids and of how this formulation is effective in describing light scattering from these systems, it will be pointed out how this interpretation can be extended to fluids. (KBE)