1. Department of Physical Chemistry, University of Geneva, 30 quai E. Ansermet, CH-1211 Geneva 4, Switzerland, Institute IRRMA, Swiss Federal Institute of Technology, PHB−Ecublens, CH-1015 Lausanne, Switzerland, and UMR 5618 CNRS, National School of Chemistry, 8 rue de l'Ecole Normale, F-34053 Montpellier, France