1. Department of Physics, Chalmers University of Technology, SE-412 96 Gothenburg, Sweden
2. Ångström Laboratory, Department of Chemistry, Uppsala University, Box 538, SE-751 21 Uppsala, Sweden
3. CNRS FR 3104, Hub de l’Energie, ALISTORE—European Research Institute, 15 Rue Baudelocque, 80039 Amiens Cedex, France