LOW-FREQUENCY DYNAMICS OF LIQUID-CRYSTALS

The nature of low-frequency (0 to 200 cm-l) dynamics of liquid crystal molecules is explored using a combination of high-resolution Raman sp ectroscopy and first principles computer simulations implemented on a parallel computer. The frequencies and displacement patterns associate d with these vibrations have been determined by diagonalisation of a d ynamical matrix. Results indicate that in the prototypical nematogen 5 CB, complex internal modes have frequencies comparable to those of lat tice modes in the low-temperature solid polymorphs. We expect this res ult is not specific to 5CB and that such behaviour is common to many l iquid-crystal systems. Such low-frequency modes may be related to the conformational changes which have been observed to occur near liquid c rystalline phase transitions. In smectic 8CB, very low-frequency featu res (near 10 cm(-1)) in the Raman spectrum are observed and are attrib uted to interlayer interactions in the smectic phase.