1. Department of Physics, Freie Universität Berlin, 14195 Berlin, Germany
2. Department of Physical Chemistry, Fritz Haber Institute of the Max Planck Society, 14195 Berlin, Germany
3. Max Planck Institute for Microstructure Physics, Weinberg 2, 06120 Halle, Germany
4. Institut für Physik, Martin-Luther-Universität Halle, 06120 Halle, Germany
5. Faculty of Mathematics and Physics, Charles University, Ke Karlovu 3, 121 16 Prague, Czech Republic