Ultrafast changes of charge density distribution in diamond after irradiation with an intense x-ray pulse (photon energy, 7.8 keV; pulse duration, 6 fs; intensity, 3×10^{19} W/cm^{2}) have been visualized with the x-ray pump-x-ray probe technique. The measurement reveals that covalent bonds in diamond are broken and the electron distribution around each atom becomes almost isotropic within ∼5 fs after the intensity maximum of the x-ray pump pulse. The 15 fs time delay observed between the bond breaking and atomic disordering indicates nonisothermality of electron and lattice subsystems on this timescale. From these observations and simulation results, we interpret that the x-ray-induced change of the interatomic potential drives the ultrafast atomic disordering underway to the following nonthermal melting.

Center of Free Electron Laser Science Deutsches Elektronen Synchrotron Notkestraße 85 22607 Germany

European XFEL GmbH Holzkoppel 4 22869 Schenefeld Germany

Faculty of Pure and Applied Sciences and Tsukuba Research Center for Energy Materials Science University of Tsukuba Tsukuba Ibaraki 305 8571 Japan

Graduate School of Pure and Applied Sciences University of Tsukuba Tsukuba Ibaraki 305 8571 Japan

Institute of Nuclear Physics Polish Academy of Sciences Radzikowskiego 152 31 342 Krakow Poland

Institute of Physics Czech Academy of Sciences Na Slovance 2 Prague 8 18221 Czech Republic

Institute of Plasma Physics Czech Academy of Sciences Za Slovankou 3 182 00 Prague 8 Czech Republic

Japan Synchrotron Radiation Research Institute Kouto 1 1 1 Sayo Hyogo 679 5198 Japan

RIKEN SPring 8 Center 1 1 1 Kouto Sayo Hyogo 679 5148 Japan

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Application of Boltzmann kinetic equations to model X-ray-created warm dense matter and plasma