Oxygen K-edge fine structures of water by x-ray Raman scattering spectroscopy under pressure conditions

X-ray Raman Scattering: A Hard X-ray Probe of Complex Organic Systems

This paper provides a review of the characterization of organic systems via X-ray Raman scattering (XRS) and a step-by-step guidance for its application. We present the fundamentals of XRS required to use the technique and discuss the main parameters of the experimental set-ups to optimize spectral and spatial resolution while maximizing signal-to-background ratio. We review applications that target the analysis of mixtures of organic compounds, the identification of minor spectral features, and the spatial discrimination in heterogeneous systems. We discuss the recent development of the direct tomography technique, which utilizes the XRS process as a contrast mechanism for assessing the three-dimensional spatially resolved carbon chemistry of complex organic materials. We conclude by exposing the current limitations and provide an outlook on how to overcome some of the existing challenges and advance future developments and applications of this powerful technique for complex organic systems.

Ion association in hydrothermal aqueous NaCl solutions: implications for the microscopic structure of supercritical water

Knowledge of the microscopic structure of fluids and changes thereof with pressure and temperature is important for the understanding of chemistry and geochemical processes. In this work we investigate the influence of sodium chloride on the hydrogen-bond network in aqueous solution up to supercritical conditions. A combination of in situ X-ray Raman scattering and ab initio molecular dynamics simulations is used to probe the oxygen K-edge of the alkali halide aqueous solution in order to obtain unique information about the oxygen's local coordination around the ions, e.g. solvation-shell structure and the influence of ion pairing. The measured spectra exhibit systematic temperature dependent changes, which are entirely reproduced by calculations on the basis of structural snapshots obtained via ab initio molecular dynamics simulations. Analysis of the simulated trajectories allowed us to extract detailed structural information. This combined analysis reveals a net destabilizing effect of the dissolved ions which is reduced with rising temperature. The observed increased formation of contact ion pairs and occurrence of larger polyatomic clusters at higher temperatures can be identified as a driving force behind the increasing structural similarity between the salt solution and pure water at elevated temperatures and pressures with drawback on the role of hydrogen bonding in the hot fluid. We discuss our findings in view of recent results on hot NaOH and HCl aqueous fluids and emphasize the importance of ion pairing in the interpretation of the microscopic structure of water.

X-ray and Electron Spectroscopy of Water

Here we present an overview of recent developments of X-ray and electron spectroscopy to probe water at different temperatures. Photon-induced ionization followed by detection of electrons from either the O 1s level or the valence band is the basis of photoelectron spectroscopy. Excitation between the O 1s and the unoccupied states or occupied states is utilized in X-ray absorption and X-ray emission spectroscopies. These techniques probe the electronic structure of the liquid phase and show sensitivity to the local hydrogen-bonding structure. Both experimental aspects related to the measurements and theoretical simulations to assist in the interpretation are discussed in detail. Different model systems are presented such as the different bulk phases of ice and various adsorbed monolayer structures on metal surfaces.

Pressure-induced dissociation of water molecules in ice VII

The neutron diffraction pattern of D₂O ice was recently measured at pressures up to 52 GPa by Guthrie et al., who proposed an octahedral interstitial model for ice at pressures above 13 GPa to account for the deviation of the observed crystal structure from that of ice VII. In this article, the octahedral interstitial model was re-examined in terms of the interstitial occupancy and X-ray Raman spectroscopy (XRS) spectra. The interstitial occupancy calculated using first-principles molecular dynamics simulations was negligibly small compared to that of the interstitial model. The oxygen K-edge spectra calculated for the interstitial model exhibited two additional low-energy peaks originating from water molecules and hydroxides that are interacting with interstitial protons, respectively, whereas these low-energy peaks were not observed in the experimentally measured spectra. These results suggest that the interstitial model cannot explain the XRS spectra of ice VII at pressures above 13 GPa and that more precise structure measurements and analyses are necessary to reveal the nature of the pressure-induced transition.

Temperature dependent soft x-ray absorption spectroscopy of liquids

A novel sample holder is introduced which allows for temperature dependent soft x-ray absorption spectroscopy of liquids in transmission mode. The setup is based on sample cells with x-ray transmissive silicon nitride windows. A cooling circuit allows for temperature regulation of the sample liquid between -10 °C and +50 °C. The setup enables to record soft x-ray absorption spectra of liquids in transmission mode with a temperature resolution of 0.5 K and better. Reliability and reproducibility of the spectra are demonstrated by investigating the characteristic temperature-induced changes in the oxygen K-edge x-ray absorption spectrum of liquid water. These are compared to the corresponding changes in the oxygen K-edge spectra from x-ray Raman scattering.

A high resolution and large solid angle x-ray Raman spectroscopy end-station at the Stanford Synchrotron Radiation Lightsource

We present a new x-ray Raman spectroscopy end-station recently developed, installed, and operated at the Stanford Synchrotron Radiation Lightsource. The end-station is located at wiggler beamline 6-2 equipped with two monochromators-Si(111) and Si(311) as well as collimating and focusing optics. It consists of two multi-crystal Johann type spectrometers arranged on intersecting Rowland circles of 1 m diameter. The first one, positioned at the forward scattering angles (low-q), consists of 40 spherically bent and diced Si(110) crystals with 100 mm diameters providing about 1.9% of 4π sr solid angle of detection. When operated in the (440) order in combination with the Si (311) monochromator, an overall energy resolution of 270 meV is obtained at 6462.20 eV. The second spectrometer, consisting of 14 spherically bent Si(110) crystal analyzers (not diced), is positioned at the backward scattering angles (high-q) enabling the study of non-dipole transitions. The solid angle of this spectrometer is about 0.9% of 4π sr, with a combined energy resolution of 600 meV using the Si (311) monochromator. These features exceed the specifications of currently existing relevant instrumentation, opening new opportunities for the routine application of this photon-in/photon-out hard x-ray technique to emerging research in multidisciplinary scientific fields, such as energy-related sciences, material sciences, physical chemistry, etc.

Role of non-hydrogen-bonded molecules in the oxygen K-edge spectrum of ice

We report the oxygen K-edge spectra of ices Ih, VI, VII, and VIII measured with X-ray Raman scattering. The pre-edge and main-edge contributions increase strongly with density, even though the hydrogen bond arrangements are very similar in these phases. While the near-edge spectral features in water and ice have often been linked to hydrogen bonding, we show that the spectral changes in the phases studied here can be quantitatively related to structural changes in the second coordination shell. Density-functional theory calculations reproduce the experimental results and support the conclusion. Our results suggest that non-hydrogen-bonded neighbors can have a significant effect also in the liquid water spectrum. We discuss the implications of the results for the actively debated interpretation of the liquid water spectrum in terms of local structure.

Probing the hydrogen-bond network of water via time-resolved soft X-ray spectroscopy

We report time-resolved studies of hydrogen bonding in liquid H(2)O, in response to direct excitation of the O-H stretch mode at 3 mum, probed via soft X-ray absorption spectroscopy at the oxygen K-edge. This approach employs a newly developed nanofluidic cell for transient soft X-ray spectroscopy in the liquid phase. Distinct changes in the near-edge spectral region (XANES) are observed, and are indicative of a transient temperature rise of 10 K following transient laser excitation and rapid thermalization of vibrational energy. The rapid heating occurs at constant volume and the associated increase in internal pressure, estimated to be 8 MPa, is manifested by distinct spectral changes that differ from those induced by temperature alone. We conclude that the near-edge spectral shape of the oxygen K-edge is a sensitive probe of internal pressure, opening new possibilities for testing the validity of water models and providing new insight into the nature of hydrogen bonding in water.

The local electronic structure of alpha-Li3N

New theoretical and experimental investigations of the occupied and unoccupied local electronic densities of states (DOS) are reported for alpha-Li(3)N. Band-structure and density-functional theory calculations confirm the absence of covalent bonding character. However, real-space full-multiple-scattering (RSFMS) calculations of the occupied local DOS find less extreme nominal valences than have previously been proposed. Nonresonant inelastic x-ray scattering, RSFMS calculations, and calculations based on the Bethe-Salpeter equation are used to characterize the unoccupied electronic final states local to both the Li and N sites. There is a good agreement between experiment and theory. Throughout the Li 1s near-edge region, both experiment and theory find strong similarities in the s-and p-type components of the unoccupied local final DOS projected onto an orbital angular momentum basis (l-DOS). An unexpected, significant correspondence exists between the near-edge spectra for the Li 1s and N 1s initial states. We argue that both spectra are sampling essentially the same final DOS due to the combination of long core-hole lifetimes, long photoelectron lifetimes, and the fact that orbital angular momentum is the same for all relevant initial states. Such considerations may be generally applicable for low atomic number compounds.