1. Department of Physics, Nanoscience Center, P.O. Box 35, University of Jyväskylä, FI-40014 Jyväskylä, Finland, Department of Physics, Tampere University of Technology, P.O. Box 692, FI-33101 Tampere, Finland, Department of Applied Physics and Competence Centre for Catalysis, Chalmers University of Technology, SE-412 96 Göteborg, Sweden, and Department of Chemistry, Nanoscience Center, P.O. Box 35, University of Jyväskylä, FI-40014 Jyväskylä, Finland