NONLINEAR RAMAN-SPECTROSCOPY OF LIQUID-CRYSTALS - POLARIZATION MEASUREMENTS AND RELAXATION PROCESSES IN 4-CYANO-4'-HEPTYLBIPHENYL (7CB)

The non-linear Raman spectrum of 4-cyano-4'-heptylbiphenyl is reported. Measurements of depolarization ratios of the vibrational bands at 2224 cm-1 (CN stretching), 1606 cm-1 (aromatic ring stretching) and 1185 cm-1 (CH aromatic in-plane deformation) are used to evaluate the order parameters ([P2] and [P4]) using a 'line model' for the molecules. The results are internally consistent and also show the same general features (including some discrepancies in [P4]) reported by other workers. Nevertheless, the results demonstrate that the technique is capable of wide application to the study of the anisotropic properties of liquid crystals and provides a valuable probe into the study of the biaxial nature of these sytems.