Pseudo-lattice vibrations in smectic phase of liquid crystals: studies on small wave number Raman spectra of 4-alkyl-4′-cyanobiphenyl Original Research Article

Small wave number Raman spectra of 4-CnH2n+1-4′-cyanobiphenyl (nCB) were measured for n=8,9,10,12. Sharp Raman bands are observed in common below about 10 cm−1 for the smectic A phase of these compounds. These bands are assigned to intermolecular vibrations along the direction perpendicular to the smectic layers each of which comprises two sublayers. Existence of such definite intermolecular vibrations has been reported previously by the present authors. We called them pseudo-lattice vibrations in smectic liquid crystals. Apparent force constant and damping factor of these vibrations are smaller for a larger n. These facts imply that for a larger n, the structural ordering in a sublayer is better, but the interaction between sublayers is smaller.