EXTENDED SMOLUCHOWSKI MODELS FOR INTERPRETING RELAXATION PHENOMENA INLIQUIDS

Interpretation of the dynamical behaviour of single molecules or colle ctive modes in liquids has been increasingly centered, in the last dec ade, on complex liquid systems, including ionic solutions, polymeric l iquids, supercooled fluids and liquid crystals. This has been made nec essary by the need of interpreting dynamical data obtained by advanced experiments, like optical Kerr effect, time dependent fluorescence sh ift experiments, two-dimensional Fourier-transform and high field elec tron spin resonance and scattering experiments like quasi-elastic neut ron scattering. This communication is centered on the definition, trea tment and application of several extended stochastic models, which hav e proved to be very effective tools for interpreting and rationalizing complex relaxation phenomena in liquids structures. First, applicatio ns of standard Fokker-Planck equations for the orientational relaxatio n of molecules in isotropic and ordered liquid phase are reviewed. In particular attention will be focused on the interpretation of neutron scattering in nematics. Next, an extended stochastic model is used to interpret time-domain resolved fluorescence emission experiments. A tw o-body stochastic model allows the theoretical interpretation of dynam ical Stokes shift effects in fluorescence emission spectra, performed on probes in isotropic and ordered polar phases. Finally, for the case of isotropic fluids made of small rigid molecules, a very detailed mo del is considered, which includes as basic ingredients a Fokker-Planck description of the molecular librational motion and the slow diffusiv e motion of a persistent cage structure together with the decay proces ses related to the changing structure of the cage.