1. Department of Chemistry, Norwegian University of Science and Technology, 7491 Trondheim, Norway
2. Istituto per i Processi Chimico Fisici del CNR (IPCF-CNR), Via G.Moruzzi, 1, 56124 Pisa, Italy
3. Scuola Normale Superiore, Piazza dei Cavalieri 7, 56126 Pisa, Italy