1. Institut de Physique des Nanostructures, Ecole Polytechnique Fédérale de Lausanne, CH-1015 Lausanne, Switzerland, T. J. Watson Research Center, IBM, Yorktown Heights, New York 10598, Departments of Chemistry and Physics and Astronomy, University of British Columbia, Vancouver, Canada, and Max-Planck Institute for Solid State Research, D-70569 Stuttgart, Germany