1. Leipzig University; Faculty of Physics and Earth Sciences; Linnéstrasse 5 04103 Leipzig Germany
2. University of Debrecen; Department of Solid State Physics; P.O. Box 400 4002 Debrecen Hungary
3. Leipzig University; Institute of Chemical Technology; Linnéstrasse 3 04103 Leipzig Germany
4. Max-Planck-Institut für Kohlenforschung; Kaiser-Wilhelm-Platz 1 45470 Mülheim an der Ruhr Germany