HIGH-RESOLUTION RAMAN-STUDY OF THE SOFT MODE IN THE INCOMMENSURATE BIS(4-CHLOROPHENYL)SULFONE

The soft optical mode of bis(4-chlorophenyl)sulfone is analysed up to the incommensurate transition temperature T-i with both a tandem of Fa bry-Perot interferometers and a triple monochromator. High resolution spectra (similar to 4 GHz) of the soft mode reveal a mean field behavi our up to T-i-4 K, but between T-i and this temperature the frequency of the soft mode saturates whereas its linewidth increases and its int ensity decreases. The occurrence of second order scattering with a ver y slow decrease of its intensity above T-i precludes the explanation o f the observed soft mode blocking by a divergence of the second order scattering. Different spectral profiles of the soft mode have been rec orded in the (bb) and (gamma gamma) polarizations, gamma being the pri ncipal dielectric axis parallel to the molecular long axis. Within sin gle mode analysis, excess intensity is found between the soft mode and the longitudinal acoustic mode in the (gamma gamma) polarization. Thi s intensity is assigned to a coupling effect between these modes where as a phason contribution is ruled out for reasons of symmetry and temp erature behaviour.