1. Institute of Experimental and Applied Physics, Kiel University, 24118 Kiel, Germany
2. Laboratoire de Physique de la Matière Condensée (PMC), CNRS, Ecole Polytechnique, Institut Polytechnique de Paris, 91120 Palaiseau, France
3. Interface Research and Catalysis, Friedrich-Alexander-Universität Erlangen-Nürnberg, 91054 Erlangen, Germany
4. European Synchrotron Radiation Facility, 38000 Grenoble, France