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