Ferguson, K. R.Bucher, M.Gorkhover, TaisBoutet, S.Fukuzawa, H.Koglin, J. E.Kumagai, Y.Lutman, A.Marinelli, A.Messerschmidt, M.Nagaya, K.Turner, J.Ueda, K.Williams, G. J.Bucksbaum, P. H.Bostedt, C.2018-04-192018-04-1920162375-2548https://depositonce.tu-berlin.de/handle/11303/7657http://dx.doi.org/10.14279/depositonce-6847In condensed matter systems, strong optical excitations can induce phonon-driven processes that alter their mechanical properties. We report on a new phenomenon where a massive electronic excitation induces a collective change in the bond character that leads to transient lattice contraction. Single large van der Waals clusters were isochorically heated to a nanoplasma state with an intense 10-fs x-ray (pump) pulse. The structural evolution of the nanoplasma was probed with a second intense x-ray (probe) pulse, showing systematic contraction stemming from electron delocalization during the solid-to-plasma transition. These findings are relevant for any material in extreme conditions ranging from the time evolution of warm or hot dense matter to ultrafast imaging with intense x-ray pulses or, more generally, any situation that involves a condensed matter-to-plasma transition.en500 Naturwissenschaften und Mathematikx-rayclusterultrafastscatteringArticle2375-2548