1. Leiden Institute of Chemistry, Gorlaeus Laboratories, Leiden University, PO Box 9502, 2300 RA Leiden, The Netherlands, Department of Physics and Astronomy, JCMB, University of Edinburgh, Edinburgh EH9 3JZ, United Kingdom, and Laboratoire Léon Brillouin, CE de Saclay, 91191 Gif-sur-Yvette Cedex, France