1. Contribution from the Condensed Matter Physics and Chemistry Department and Optics and Fluid Dynamics Department, Risø National Laboratory, POB 49, DK-4000 Roskilde, Denmark, UNI-C, Olof Palmes Allé 38, DK-8200 Aarhus N, Denmark, and Chemistry Laboratory IV, Department of Chemistry, University of Copenhagen, Universitetsparken 5, DK-2100 Copenhagen Ø, Denmark