1. Institute of Applied Physics, University of Erlangen-Nuremberg, Staudtstr. 7, Bldg. A3, 91058 Erlangen, Germany, Institute of Physics and Center for Nanotechnology, University of Münster, Wilhelm-Klemm-Str. 10, 48149 Münster, Germany, and Institute for Organic and Biomolecular Chemistry, University of Göttingen, Tammannstr. 2, 37077 Göttingen, Germany