1. Center for Free-Electron Laser Science/DESY, Notkestraße 85, D-22607 Hamburg, Germany
2. Institute of Optics and Atomic Physics, TU Berlin, Hardenbergstraße 36, 10623 Berlin, Germany
3. Helmholtz-Zentrum Berlin, Albert-Einstein-Straße 15, 12489 Berlin, Germany
4. Physikalisch-Technische Bundesanstalt, Abbestraße 2-12, 10587 Berlin, Germany
5. Institute for Experimental Physics, TU Bergakademie Freiberg, Leipziger Straße 23, 09599 Freiberg, Germany