1. Institute of Nanostructure and Solid State Physics, University of Hamburg, Luruper Chaussee 149, 22761 Hamburg, Germany
2. The Hamburg Centre for Ultrafast Imaging, Luruper Chaussee 149, 22761 Hamburg, Germany
3. Deutsches Elektronen-Synchrotron DESY, Notkestr. 85, 22607 Hamburg, Germany