1. ITODYS-CNRS, Université Paris Diderot, F-75005 Paris, France, CQFM, IST, Technical University of Lisbon, P-1049-001 Lisboa, Portugal, IRCELYON-CNRS, 2, avenue A. Einstein F-69626 Villeurbanne cedex, France, and Department of Chemical Physics, The Weizmann Institute, Rehovot 76100, Israel