1. Center for Free-Electron Laser Science, DESY, Notkestrasse 85, 22607 Hamburg, Germany
2. Department of Physics, University of Hamburg, Jungiusstrasse 9, 20355 Hamburg, Germany
3. The Hamburg Centre for Ultrafast Imaging, Luruper Chaussee 149, 22761 Hamburg, Germany
4. SLAC National Accelerator Laboratory, Stanford University, 2575 Sand Hill Road, Menlo Park, California 94025, USA