Non-isotropic Atomic Motion in Warm Dense Carbon

X-ray Free Electron Laser (XFEL) radiation can transform carbon into a warm dense matter state. Two X-ray pulses were used; the first as pump to reach the warm dense matter state and the second as probe performing X-ray diffraction. The experiment was performed at the SACLA XFEL facility at the beamline 3 experimental hutch 5. The samples were polycrystalline diamond. The pump and probe photon energies were 7 and 10.5 keV, respectively, and the delay between the X-ray pulses was varied from 0 to 286 fs. To provide a range of energy densities, the X-ray focus was adjusted between 150 nm and 1 μm. The (111), (220) and (311) diffraction peaks were observed. The intensity of each diffraction peak decreased with time indicating a disordering of the crystal lattice. From a Debye-Waller analysis, the root-mean-square (rms) atomic displacement perpendicular to particular lattice planes are calculated. At higher fluences, the rms atomic displacement perpendicular to the (111) planes is significantly larger than that perpendicular to the (220) or (311) planes. By accepting two successive XFEL pulses at a time delay of 33 ms, graphite (002) diffraction was observed beginning at a threshold dose of 1.5 eV/atom. The experimental results will be compared with calculations using a hybrid model based on tight-binding molecular dynamics.