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.

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The structure factors of diamond were determined by synchrotron radiation X-ray powder diffraction at 800 K at sin / 2.2 Å À1 reciprocal resolution. The structure factors were estimated using six powder profiles measured on beamline BL02B2 at SPring-8 (Hyogo, Japan). A high reciprocal resolution at sin / 2.2 Å À1 was required to reveal the temperature dependence of the charge density, due to the high Debye temperature of D = 1860 K of diamond. Wide 2 angle data with the highest counting statistics are crucial for accurate data analysis. The periodic noise of every six-pixel step was observed in the highest counting statistics imaging plate (IP) data scanned by a BAS2500 IP scanner. It was found that the noise originated from the six-sided polygonal mirror in the scanner. The intensity fluctuation at every six-pixel step was also found in the Fourier series expansion of the powder profiles. The ratio of the maximum fluctuation was estimated as 0.4% by summing all six-pixel step data. The powder profiles were corrected by multiplying the ratios. The intensity fluctuation in the background region was reduced to less than 50% of the uncorrected data. The weak 888 Bragg reflection, with an intensity of 0.005% of that of the 111 Bragg reflection at 800 K, was readily observed in the corrected data. Finally, the structure factors determined at 800 K were successfully applied to a charge-density study by multipole modelling. The reliability factors and multipole parameters at 800 K are in agreement with those at 300 K. The differences in the charge density at the bond midpoint and r 2 at the bondcritical point were less than 1% and 2%, respectively.

Carbon has attracted wide attention owing to the main component of organics and their several functional allotrope, graphite, graphene, fullerene, and diamond. Diamond is the first structurally characterized material by L. Bragg. We have measured accurate powder diffraction data of diamond at the third-generation synchrotron X-ray source, SPring-8 and PETRA-. We determined accurate charge densities by maximum entropy method (MEM) and multipole refinement from the data [1,2]. We also observed the core deformation of carbon atom from the charge density [3]. These studies confirm that powder diffraction data of diamond is suitable for accurate structural study. The fact suggests that the response of electron distribution under external fields such as temperature and pressure can be precisely revealed by using the powder data of diamond. In this study, we have investigated the temperature and pressure dependence of charge density for diamond from powder data at SPring-8. Multi-temperature synchrotron powder diffraction data were collected at BL02B2 beamline, SPring-8. The data were measured at 30-800 K using a wave length 0.32905 Å. We have observed bragg reflection at 2.57 Å-1. High pressure powder diffraction experiments using a diamond anvil cell (DAC) were carried out at BL02B1, SPring-8. We measured high pressure powder data up to 3.58 GPa using a wave length 35 keV. We have observed temperature dependence of accurate charge densities of Diamond at 300, 600 and 800K. The covalent bonding in diamond was clearly recognized and was consistent with the previous studies. We also observed that relative intensities of low order reflections, such as 111 and 220, are changed with increasing pressure.[1] E. Nishibori et al

粉末 X 線回折による精密構造解析 Tomoaki SASAKI, Yuka DEGUCHI, Hidetaka KASAI and Eiji NISHIBORI: Accurate Charge Density Study of Aluminum from Synchrotron X-ray Powder Diffraction X-ray charge density is one of the most information rich observables in natural science. We have developed the diffractometers and measurement techniques for X-ray charge density study at one of the third generation synchrotron radiation source SPring-8. Very small amounts of aspherical electron distribution of a pure metal aluminum were observed in the static deformation density. A signal to noise ratio of high temperature powder diffraction data was improved by the data correction based on Fourier analysis.