1. Max Planck Institute for the Structure and Dynamics of Matter, Centre for Ultrafast Imaging, Luruper Chaussee 149, 22761 Hamburg, Germany
2. Department of Physics, University of Hamburg, Jungiusstraße 9-11, 20355 Hamburg, Germany
3. Departments of Chemistry and Physics, University of Toronto, 80 St. George Street, Toronto, Ontario M5S 3H6, Canada