1. Laboratory for Ultrafast Microscopy and Electron Scattering, École Polytechnique Fédérale de Lausanne, CH-1015 Lausanne,
Switzerland
2. JILA, University of Colorado, 440 UCB, Boulder,
Colorado 80309-0440, USA
3. Laboratory of Ultrafast Spectroscopy, École Polytechnique Fédérale de Lausanne, CH-1015 Lausanne,
Switzerland
4. Particle Beam Physics Laboratory, Department of Physics and Astronomy, UCLA, Los Angeles, California 90095,
USA
5. Ludwig-Maximilians-Universität München, Am
Coulombwall 1, 85748 Garching, Germany