1. Institute for Nanostructure and Solid-State Physics, Department of Physics, University of Hamburg and Center for Free-Electron Laser Science, Luruper Chaussee 149, 22761 Hamburg, Germany
2. Physical Chemistry and Physics departments, University of Hamburg, Luruper Chaussee 149, 22761 Hamburg, Germany
3. The Hamburg Centre for Ultrafast Imaging, University of Hamburg, Luruper Chaussee, 149, 22761 Hamburg, Germany
4. Physics and Chemistry of Nanostructures, Department of Chemistry, Ghent University, Krijgslaan 281 - S3, B-9000 Gent, Belgium
5. Center for Nano and Biophotonics, Ghent University, Technologiepark Zwijnaarde 15, B-9052 Gent, Belgium