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 ${\rm C}_{n}{\rm H}_{2n+2}$/urea inclusion compounds with $n = 12, 14, 17\mbox{-}19$. 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. However, quasi-elastic components with different widths are observed for different polarizations. In one of them the characteristics of the scattered light suggest that the quasi-elastic 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.