ORIENTATIONAL DYNAMICS IN THE ISOTROPIC-PHASE OF A NEMATIC MIXTURE - SUBPICOSECOND TIME-RESOLVED OPTICAL KERR-EFFECT EXPERIMENTS ON ZLI-1167 LIQUID-CRYSTAL

We have investigated the orientational dynamics of the ternary mixture of propyl, pentyl and heptyl cyano-cyclohexyls (ZLI-1167) in the isot ropic phase over a wide temperature range (372-453 K) by means of tran sient optical Kerr effect spectroscopy. The orientational dynamics of ZLI-1167 isotropic phase shows a complex relaxation pattern where seve ral dynamical processes take place. We substantially observed three ti me scales of relaxation: fast (from hundreds of fsec to few psec), int ermediate (from tens to hundreds of psec) and slow (from hundreds of p sec to few nsec). The slow orientational relaxation shows a temperatur e dependence characterized by strong pre-transitional effects. The cri tical slowing down of these relaxation times can be correctly interpre ted by Landau-de Gennes theory. The fast and intermediate decay times show a different temperature dependence without any observable pre-tra nsitional effects. We suggest an interpretation of the experimental re sults in term of averaged single molecule dynamics and collective dyna mics.