1. Department of Physics; Technical University of Denmark, Building 307; 2800 Kgs. Lyngby Denmark
2. Present address: Institute of Surface Chemistry and Catalysis; Ulm University; Albert-Einstein-Allee 47 89069 Ulm Germany
3. Present address: Department of Materials; Imperial College London, Royal School of Mines Building; Prince Consort Rd. London SW7 2AZ UK