BRILLOUIN STUDY OF ACOUSTIC MODES IN ISOTROPIC, SMECTIC-A, AND SMECTIC-C-ASTERISK PHASES IN A FERROELECTRIC LIQUID-CRYSTAL

This work concerns the study of collective modes in a smectic liquid c rystal in the hypersonic frequency range. Special attention is paid to the determination of the significant anisotropy of the quasilongitudi nal mode and to its temperature dependence, namely, near the isotropic -smectic-A phase transition. The interpretation is provided in terms o f the pseudocrystalline model by means of effective stiffness and visc osity tensors. A weak quasitransverse mode has ken detected: the angul ar variations of its frequency and of its intensity are in good agreem ent with the predictions of our model. We confirm a universal behavior for nematic and smectic phases at high frequencies: in the hypersonic regime the orientational order of the molecules seems to play a more important role than the smectic ordering. A quantitative analysis of t he angular and temperature dependence of the inelastically scattered i ntensity is done: the observed intensity anomalies are well described by our model. The smectic-C-smectic-A phase transition is investigate d by means of Brillouin scattering: only very small anomalies are foun d.