Raman scattering: Investigation of nematic and smectic ordering

Abstract

Quantitative measurements of vibrational Raman depolarization ratios have been used to study molecular orientational order in nematic and for the first time, in smectic liquid crystals. Temperature dependence of the depolarization ratios of the terminal C≡N and the central biphenyl link vibrational bands of two alkylcyanobiphenyl compounds (CB_n) and a cyclohexane derivative (PCH₇) has been investigated extensively. Two experimental technics have been used: a conventional triple monochromator and the Raman microprobe to overcome the spurious depolarization due to the long wave director fluctuations. The orientational order parameters <P₂≳ and <P₄≳ were then calculated. The results have been discussed assuming repulsive forces are dominant for the CB_n compounds and using the Maïer–Saupe theory in the case of PCH₇.