1. Fachbereich Physik, Freie Universität Berlin, Arnimallee 14, 14195 Berlin, Germany ()
2. IBM Zurich Research Laboratory, Säumerstrasse 4/Postfach, 8803 Rüschlikon, Switzerland
3. Department of Physics and Astronomy, Ohio University, Athens, OH 45701, USA
4. Paul-Drude-Institute, Hausvogteiplatz 5-7, 10117 Berlin, Germany
5. Department of Chemistry, University of California Riverside, 900 University Avenue, Riverside, CA 92521-0403, USA