This project received funding from the European Union’s Horizon 2020 research and innovation programme under the Marie Sklodowska-Curie grant agreement no. 641789 MEDEA and the Italian Ministry of Research (Project FIRB no. RBID08CRXK). K.U. acknowledges support from the X-ray Free Electron Laser Utilization Research Project and the X-ray Free Electron Laser Priority Strategy Program of the Ministry of Education, Culture, Sports, Science and Technology of Japan (MEXT), from the Cooperative Research Program ‘Network Joint Research Center for Materials and Devices: Dynamic Alliance for Open Innovation Bridging Human, Environment and Materials’, from the bilateral project CNR-JSPS ‘Ultrafast science with extreme ultraviolet Free Electron Lasers’, and from the IMRAM project for international co-operation. R.F. and J.M. thank the Swedish Research Council (VR) and the Knut and Alice Wallenberg Foundation for financial support. E.V.G. acknowledges support from the Foundation for the Advancement of Theoretical Physics and Mathematics ‘BASIS’. M. Meyer and T.M. acknowledge support from the Deutsche Forschungsgemeinschaft (DFG) under grant no. SFB925/1. A.A.L. was supported by the US Department of Energy contract no. DE-AC02-76SF00515. Research at Louisiana State University was supported by the US Department of Energy, Office of Science, Basic Energy Sciences, under contract no. de-sc0010431. Portions of this research were conducted with high performance computing resources provided by Louisiana State University (http://www.hpc.lsu.edu) and by Louisiana Optical Network Infrastructure (http://hpc.loni.org). G.S. acknowledges useful discussions about the simulations and the data analysis with T. Pfeifer and M. Kübel. We acknowledge L. Foglia, A. Simoncig and M. Coreno for valuable discussions.