ON THE DETERMINATION OF ORIENTATIONAL ORDER PARAMETERS IN LIQUID-CRYSTALS FROM RAMAN-SCATTERING - CB9 REVISITED

Orientational order parameters P-2 and P-4 have been determined for th e nematic and smectic phases of CB9 (4-nonyl-4'-cyanobiphenyl) from Ra man scattering intensity measurements. The derivations of P-2 and P-4 from Raman intensities have been made using uniaxial or biaxial Raman tensor assumptions, but the orientational distribution function is wel l peaked at beta = 0 only when a uniaxial tensor is used. The opposite choice of a biaxial Raman tensor and its consequences are discussed. Negative values of P-4 found in our study are shown not to affect seri ously the orientational distribution. Furthermore, it is shown that su ch negative values, if not explained by microscopic theories, are allo wed when a Landau-type phenomenological approach is used. Nevertheless , it is once more emphasized that local field effects should be taken into account.