Arwen R. Pearson, Briony A. Yorke, Matteo Levantino, Marco Cammarata, Diana C. F. Monteiro, Department of Physics and Chemistry, Università degli studi di Palermo - University of Palermo, University of Hamburg, University of Leeds, Institut de Physique de Rennes (IPR), Université de Rennes (UR)-Centre National de la Recherche Scientifique (CNRS), ML acknowledges the financial support of the University of Palermo (FFR program 2012/2013). ARP and BAY are supported by the excellence cluster ‘The Hamburg Center for Ultrafast Imaging—Structure, Dynamics and Control of Matter at the Atomic Scale’ of the Deutsche Forschungsgemeinschaft. DCFM is supported by the Wellcome Trust (096684/Z/11/Z). ARP and ML thank the EU-COST action CM1306 for additional support., Levantino, M, Yorke, BA, Monteiro, DCF, Cammarata, M, Pearson, AR, Université de Rennes 1 (UR1), and Université de Rennes (UNIV-RENNES)-Université de Rennes (UNIV-RENNES)-Centre National de la Recherche Scientifique (CNRS)