1. Department of Chemistry and CSGI, University of Florence, Via della Lastruccia 3, I-50019 Sesto Fiorentino, Firenze, Italy, VTT Processes, P. O. Box 1601, 02044 VTT, Finland, and Laboratoire Léon Brillouin, CEA-CNRS, CEN-Saclay, 91191 Gif-sur-Yvette, France