1. Max-Planck-Institut für Struktur und Dynamik der Materie, Luruper Chaussee 149, D-22761 Hamburg, Germany
2. Center for Free-Electron Laser Science, Luruper
Chaussee 149, D-22761 Hamburg, Germany
3. The Hamburg Centre for Ultrafast Imaging, Universität Hamburg, Luruper Chaussee 149, D-22761 Hamburg, Germany
4. Department of Physics, Harvard University, Cambridge, Massachusetts 02138, United States