1. Institute for Quantum Electronics, Eidgenössische Technische Hochschule (ETH) Zürich, CH-8093 Zurich, Switzerland
2. Paul Scherrer Institut, CH-5032 Villigen, Switzerland
3. Laboratory for Ultrafast Microscopy and Electron Scattering, ICMP, Ecole Polytechnique Fédérale de Lausanne (EPFL), CH-1015 Lausanne, Switzerland
4. Department of Physics, University of Zurich, CH-8057 Zurich, Switzerland