Direct Observation of Picosecond Reorientation of Molecules in the Isotropic Phases of Nematogens

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Abstract
We present here an experimental investigation of individual molecular movements inside the isotropic phase of nematogens, by means of Optical Kerr EKect techniques. They give rise to a fast response of amplitude C, in the picosecond range, which is evidenced. Its time constant is evaluated and compared to that of molecules prefigurating liquid crystals but showing no nematic phase. Moreover, we present in the case of p-methoxybenzylidene-p-n-butylanili (MBBA), the first direct measurement of the ratio CF/Cs, where C, is the amplitude of the well known slow response, induced by the order parameter fluctuations.