1. Section for Surface Physics and Catalysis, Department of Physics, Technical University of Denmark, DK-2800 Kgs. Lyngby, Denmark
2. Science Institute and Faculty of Physical Sciences, VR-III, University of Iceland, IS-107 Reykjavik, Iceland
3. Institute of Surface Chemistry and Catalysis, Ulm University, D-89069 Ulm, Germany
4. Department of Materials, Imperial College London, Royal School of Mines, London SW7 2AZ, U.K.