1. Department of Physics, University of Florence and I. N. F. M., v. G. Sansone 1, 50019 Sesto Fiorentino, Firenze, Italy, Department of Physics, University of Messina and I. N. F. M., Salita Sperone 31, 98010 S. Agata, Messina, Italy, Department of Chemistry, University of Florence and C. S. G. I., V. G. Capponi 9, 50121 Firenze, Italy, and Laboratoire Léon Brillouin, CEA-CNRS Saclay, 91191 Gif sur Yvette Cédex, France