We report on the microscopic structure of water at sub-and supercritical conditions studied using X-ray Raman spectroscopy, ab initio molecular dynamics simulations, and density functional theory. Systematic changes in the X-ray Raman spectra with increasing pressure and temperature are observed. Throughout the studied thermodynamic range, the experimental spectra can be interpreted with a structural model obtained from the molecular dynamics simulations. A spatial statistical analysis using Ripley's K-function shows that this model is homogeneous on the nanometer length scale. According to the simulations, distortions of the hydrogenbond network increase dramatically when temperature and pressure increase to the supercritical regime. In particular, the average number of hydrogen bonds per molecule decreases to ≈0.6 at 600°C and p = 134 MPa.supercritical water | water structure | X-ray scattering | X-ray scattering spectroscopy
The heat-induced desorption and adsorption of the proteins lysozyme, ribonuclease A, bovine serum albumin, and fibronectin at protein layers was investigated in two different environments: pure buffer and protein solution. Using two different environments allows us to distinguish between thermodynamic and kinetic mechanisms in the adsorption process. We observed a desorption in buffer and an adsorption in protein solution, depending upon protein properties, such as size, stability, and charge. We conclude that the desorption in buffer is mainly influenced by the mobility of the proteins at the interface, while the adsorption in protein solution is driven by conformational changes and, thereby, a gain in entropy. These results are relevant for controlling biofilm formation at solid-liquid interfaces.
Ultraviolet photoelectron spectroscopy (UPS) was used to investigate size selected Ag 923AE9 and Ag 55 clusters which were softlanded on a clean graphite substrate (HOPG) at 100 and 50 K, respectively. With increasing cluster coverage closer to the centre of the deposition spot a continuous change of the d-band signal is observed. Differences in the fine structure of the d-band and comparison to UPS spectra of clusters grown at nanopits on HOPG show that the clusters in the centre of the deposition spot coalesced. However, Ag 55 spectra measured at the rim of the deposition spot indicate that the clusters stay separated in regions of lower coverage for a deposition temperature of 50 K. This is corroborated by scanning tunnelling microscopy (STM) images measured at 5 K using 1 monolayer (ML) Xe to fix the Ag 55 clusters to the substrate, thus making them observable with STM. By comparison to UPS data taken on different sample positions in a 1 Â 1 mm 2 grid it was determined that at the rim of the deposition spot the coverage of 30 clusters per 100 Â 100 nm 2 was low enough for an UPS measurement of single separated Ag 55 clusters. Differences in the spectra for the largest coverage of Ag 55 and Ag 923 clusters in the deposition spot centre indicate that the resulting Ag film has a partial (111) orientation for the deposition of Ag 55 at 50 K whereas it is mostly polycrystalline for Ag 923 deposited at 100 K.
X-ray diffraction experiments at 80 K show that when silicon is compressed under hydrostatic conditions the intermediate high-pressure phases are bypassed leading to a direct transformation to the simple hexagonal structure at 17 GPa. A maximum entropy analysis of the diffraction patterns reveals dramatic alterations in the valence electron distribution from tetrahedral covalent bonding to localization in the interstitial sites and along the one-dimensional silicon atom chain running along adjacent hexagonal layers. Changes in the orbital character of the unoccupied states are confirmed using X-ray Raman scattering spectroscopy and theoretical Bethe-Salpeter equation calculations. This is the first direct observation indicating that the silicon valence electrons in 3s and 3p orbitals are transferred to the 3d orbitals at high density which proves that electrons of compressed elemental solids migrate from their native bonding configuration to interstitial regions.
, and the semiconducting BaSi 2 . Here, non-resonant x-ray Raman scattering is used to investigate confinement effects on the shape of the giant resonance in the vicinity of the Ba N IV,V -edge. The distinct momentum transfer dependence of the spectra is analyzed and discussed. The measurements are confronted with calculations of the giant resonance within time-dependent local density approximation in the dipole limit. No modulation of the giant resonance's shape for Ba atoms confined in different local environments was observed, unlike predicted by the calculations. The absence of such shape modulation for complex Ba/Si compounds is discussed providing important implications for further studies of giant resonance phenomena utilizing both theory and experiment.
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