1. Département de Chimie, Ecole Normale Supérieure, UMR 8640 Pasteur, 24 rue Lhomond, 75231 Paris Cedex 05, France, Departament de Física i Enginyeria Nuclear, Universitat Politècnica de Catalunya, Campus Nord B4-B5, Barcelona 08034, Spain, Max Born Institut für Nichtlineare Optik und Kurzzeitspektroskopie, Max Born Straβe 2A, D-12489 Berlin, Germany, Department of Chemistry and Biochemistry, University of Colorado, Boulder, Colorado 80309-0215