1. Center for Molecular Movies and Department of Chemistry, Technical University of Denmark (DTU), Building 207, DK-2800 Kgs. Lyngby, Denmark, Niels Bohr Institute, University of Copenhagen, Universitetsparken 5, DK-2100 Copenhagen Ø, Denmark, and Department of Physics, Norwegian University of Science and Technology, Høgskoleringen 5, N-7491 Trondheim, Norway