LIGHT-SCATTERING BY LOW-FREQUENCY EXCITATIONS IN QUASI-PERIODIC N-ALKANE UREA ADDUCTS/

High-resolution light scattering spectroscopy has been performed on a series of n-alkane CnH2n+2/urea inclusion compounds with n = 12, 14, 1 7-19. The elastic constants of these incommensurate composite crystals have been determined and found to be independent of the chain length of the guest molecules. In addition, no extraneous acoustic-like mode appeared in all spectra. Hen-ever, quasi-elastic components with diffe rent widths are observed for different polarizations. In one of them t he characteristics of the scattered light suggest that the quasi-elast ic broadening could be generated by a sliding mode, whereas the other detected component is assigned to the reorientational motions of the n -alkane chains about their long axis.