RT Journal Article SR Electronic T1 Transient lattice contraction in the solid-to-plasma transition JF Science Advances JO Sci Adv FD American Association for the Advancement of Science SP e1500837 DO 10.1126/sciadv.1500837 VO 2 IS 1 A1 Ferguson, Ken R. A1 Bucher, Maximilian A1 Gorkhover, Tais A1 Boutet, Sébastien A1 Fukuzawa, Hironobu A1 Koglin, Jason E. A1 Kumagai, Yoshiaki A1 Lutman, Alberto A1 Marinelli, Agostino A1 Messerschmidt, Marc A1 Nagaya, Kiyonobu A1 Turner, Jim A1 Ueda, Kiyoshi A1 Williams, Garth J. A1 Bucksbaum, Philip H. A1 Bostedt, Christoph YR 2016 UL http://advances.sciencemag.org/content/2/1/e1500837.abstract AB In 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.