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 isotropic phase over a wide temperature range (372-453 K) by means of transient optical Kerr effect spectroscopy. The orientational dynamics of ZLI-1167 isotropic phase shows a complex relaxation pattern where several dynamical processes take place. We substantially observed three time scales of relaxation: fast (from hundreds of fsec to few psec), intermediate (from tens to hundreds of psec) and slow (from hundreds of psec to few nsec). The slow orientational relaxation shows a temperature dependence characterized by strong pre-transitional effects. The critical slowing down of these relaxation times can be correctly interpreted by Landau-de Gennes theory. The fast and intermediate decay times show a different temperature dependence without any observable pre-transitional effects. We suggest an interpretation of the experimental results in term of averaged single molecule dynamics and collective dynamics.