Time-resolved X-ray diffraction measurements were carried out at the XPP endstation at LCLS. Use of the Linac Coherent Light Source (LCLS), SLAC National Accelerator Laboratory, is supported by the US Department of Energy, Office of Science, Office of Basic Energy Sciences under contract number DE-AC02-76SF00515. Preparatory static diffraction measurements were performed at the X04SA beamline of the Swiss Light Source. We acknowledge financial support by the NCCR Molecular Ultrafast Science and Technology (NCCR MUST), a research instrument of the Swiss National Science Foundation (SNSF). E.A. acknowledges support from the ETH Zurich Postdoctoral Fellowship Program and from the Marie Curie Actions for People COFUND programme. E.M.B. acknowledges funding from the European Commission’s Seventh Framework Programme (FP7/2007-2013) under grant agreement number 290605 (PSI-FELLOW/COFUND). M.P. acknowledges support from NCCR MARVEL, funded by the SNSF.

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Nature thanks P. G. Evans, M. Münzenberg and S. Sharma for their contribution to the peer review of this work.