Europhys. Lett, 43 (5), pp. 546-551 (1998)
Light scattering by low-frequency excitations in
J. Ollivier 1, C. Ecolivet 1, S. Beaufils 1, F. Guillaume 2 and T. Breczewski 3
1 Groupe Matière Condensée et Matériaux, URA CNRS 804,
Université de Rennes
Campus de Beaulieu, Rennes 35042, France
2 Laboratoire de Spectroscopie Moléculaire et Cristalline, URA CNRS 124
Université de Bordeaux 1 - 351 cours de la Libération, 33405 Talence cedex, France
3 Departemento de Física de la Materia Condensada, Facultad de Ciencias
Universidad del Pais Vasco - Apdo 644, Bilbao, Spain
(received 30 April 1998; accepted 13 July 1998)
PACS. 61.44Fw - Incommensurate crystals.
PACS. 63.20 - Phonons in crystal lattices.
PACS. 78.35 - Brillouin and Rayleigh scattering; other light scattering.
High-resolution light scattering spectroscopy has been performed on a series of n-alkane /urea inclusion compounds with . 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.