Diamond bulk irradiated with a free-electron laser pulse of 6100 eV photon energy, 5 fs duration, at the ~19-25 eV/atom absorbed doses, is studied theoretically on its way to warm dense matter state. Simulations with our hybrid code XTANT show disordering on sub-100 fs timescale, with the diffraction peak (220) vanishing faster than the peak (111). The warm dense matter formation proceeds as a nonthermal damage of diamond with the band gap collapse triggering atomic disordering. Short-living graphite-like state is identified during a few femtoseconds between the disappearance of (220) peak and the disappearance of (111) peak. The results obtained are compared with the data from the recent experiment at SACLA, showing qualitative agreement. Challenges remaining for the accurate modeling of the transition of solids to warm dense matter state and proposals for supplementary measurements are discussed in detail.

Center for Free Electron Laser Science DESY Hamburg 22607 Germany

Institute of Nuclear Physics Polish Academy of Sciences Kraków 31 342 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 Prague 8 18200 Czech Republic

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Density functional tight binding approach utilized to study X-ray-induced transitions in solid materials

Dependence of nonthermal metallization kinetics on bond ionicity of compounds