1. Center for Free-Electron Laser Science, DESY, Notkestraße 85, D-22607 Hamburg, Germany
2. The Hamburg Centre for Ultrafast Imaging, University of Hamburg, Luruper Chaussee 149, D-22761 Hamburg, Germany
3. Department of Physics, University of Hamburg, Jungiusstraße 9, D-20355 Hamburg, Germany
4. Department of Physics and Astronomy, Aarhus University, DK-8000 Aarhus C, Denmark