1. Faculty of Physics; Ludwig-Maximilians-Universität; Geschwister-Scholl-Platz 1 80539 München Germany
2. Walter Schottky Institut and Physics Department; Technische Universität München; Am Coulombwall 4 85748 Garching Germany
3. Faculty of Science; Department of Chemistry; University of Paderborn; Warburger Straße 100 33098 Paderborn Germany