A PHYSICAL INSIGHT INTO THE ELASTIC INCOHERENT STRUCTURE FACTOR

The Elastic Incoherent Structure Factor (EISF) turns out to be a basic quantity of fundamental importance in quasielastic neutron scattering. It is introduced as the long-time limit of the intermediate scattering function and it is related to the region of the space accessible to the scatterers. Thus it yields direct information about the geometry of their motions. Several examples illustrate how its determination from the experimental spectra enables one to build a particular dynamical model and also how, depending on the instrument, different aspects of the motions are revealed. The effects of the presence of multiple scattering are considered. Finally, recent studies of metastable glassy phases are presented.