1. Department of Chemistry, University of Copenhagen, Universitetsparken 5, DK-2100 Copenhagen, Denmark, Division of Nuclear Physics, Department of Physics, Lund University, P.O. Box 118, SE-221 00 Lund, Sweden, and Institut de Recherches sur la Catalyse et l’Environnement de Lyon, UMR 5256 CNRS/Université Lyon 1, 2 avenue Albert Einstein, F-69629 Villeurbanne Cedex, France