Non‐linear 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.