1. Center for Free-Electron Laser Science CFEL, Deutsches Elektronen-Synchrotron DESY, Notkestr. 85, 22607 Hamburg, Germany
2. Center for Ultrafast Imaging, Universität Hamburg, Luruper Chaussee 149, 22761 Hamburg, Germany
3. Department of Physics, Universität Hamburg, Luruper Chaussee 149, 22761 Hamburg, Germany
4. Department of Physics and Astronomy, University College London, Gower Street, WC1E 6BT London, United Kingdom