1. Department of Chemistry, University of Florence, Via della Lastruccia 3, 50019 Sesto Fiorentino, Italy, Laboratory SCR/MI-INSA, INSA-Rennes, 20 av. des buttes de Coësmes, CS 14315, 35043 Rennes Cedex, France, and 1. Physikalisches Institut, Universität Stuttgart, Pfaffenwaldring 57, D-70550 Stuttgart, Germany