1. Contribution from the Department of Materials and Interfaces, Weizmann Institute of Science, Rehovot 76100, Israel, Department of Solid State Physics, Risø National Laboratory, DK-4000 Roskilde, Denmark, Niels Bohr Institute, H. C. Øersted Laboratory, Universitetsparken 5, DK-2100 Copenhagen, Denmark, and Department of Chemistry & Molecular Biology and Skaggs Institute for Chemical Biology, The Scripps Research Institute, La Jolla, California 92037