Raman investigation on the lattice modes (8-200 ${\mathrm{cm}}^{-1\mathrm{}}$) of several smectic phases in terephtal-bis-butylaniline (TBBA) reveals significant spectral differences. Softening of the lowest lattice 19-${\mathrm{cm}}^{-1\mathrm{}}$ mode, observed very near to the solid-smectic-$B$ phase transition, is correlated with the gradual release of the smectic inter-layer glide motion. The Raman spectra add new insight into the ordering within the layers of the metastable smectic phases and also indicate the existence of molecular dynamic rotations in the smectic-$B$ phase.

• Received 3 June 1974

DOI:https://doi.org/10.1103/PhysRevA.12.681