COLLECTIVE EXCITATIONS IN DISORDERED-SYSTEMS

After an introduction into the field of collective excitations in diso rdered matter and into neutron inelastic scattering (NIS) techniques u sed for their investigation, results obtained by NIS experiments on cl assical fluids and amorphous solids are reviewed. It is concluded that a reasonably good understanding has been acquired during the years co ncerning the dispersion of excitations in liquid metals and rare gas f luids and partly also in metallic glasses. However, the damping of the se modes and the collective excitations in fluids and in amorphous sol ids with other than metallic or pure molecular bonds still need to be investigated experimentally in mom detail until a comparable understan ding of the collective movements and their decay in these systems has been gained and theoretical predictions can be tested on a quantitativ e basis. With a few exceptions systematic studies under favorable expe rimental conditions (dedicated instruments) and high quality data are lacking up to now.