1. Department of Chemistry, The University of Arizona, Tucson, Arizona 85721-0041, Institut für Festkörperphysik, Technische Universität Graz, Petersgasse 16, A-8010 Graz, Austria, Department of Chemistry and Biochemistry, Georgia Institute of Technology, Atlanta, Georgia 30332-0400, and Physics Department, University of Antwerp (UIA), Universiteitsplein 1, B-2610 Antwerpen (Wilrijk), Belgium