1. Walter Schottky Institut and Physik Department, Technical University of Munich, Am Coulombwall 4, 85748 Garching, Germany
2. Paul-Drude-Institut für Festkörperelektronik, Hausvogteiplatz 5-7, 10117 Berlin, Germany
3. Department of Chemistry, Ludwig-Maximilians-University Munich, Butenandtstr. 5, 81377 Munich, Germany