1. Department of Physics, University of Basel, Klingelbergstrasse. 82, CH-4056 Basel, Switzerland, Department of Chemistry, University of Basel, St. Johanns-Ring 19, CH-4056 Basel, Switzerland, Forschungszentrum Karlsruhe GmbH, Institute for Nanotechnology, P.O. Box 3640, D-76021 Karlsruhe, Germany, and Department of Chemistry, University of Durham, Durham DH1 3LE, U.K.