1. F. Besenbacher and I. Stensgaard, Center for Atomic-scale Materials Physics, Institute of Physics and Astronomy, University of Aarhus, DK-8000 Aarhus C, Denmark.
2. I. Chorkendorff and J. K. Nørskov, Center for Atomic-scale Materials Physics, Department of Physics, Technical University of Denmark, DK-2800 Lyngby, Denmark.
3. B. S. Clausen and A. M. Molenbroek, Haldor Topsøe Research Laboratories, Nymøllevej 55 DK-2800 Lyngby, Denmark.
4. B. Hammer, Institute of Physics, Aalborg University, DK-9220 Aalborg Øst, Denmark.