Christoph J. Sahle

Microscopic structure of water at elevated pressures and temperatures

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 hydrogen-bond 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.

Saturation behavior in X-ray Raman scattering spectra of aqueous LiCl.

We report a study on the hydrogen-bond network of water in aqueous LiCl solutions using X-ray Raman scattering (XRS) spectroscopy. A wide concentration range of 0-17 mol/kg was covered. We find that the XRS spectral features change systematically at low concentrations and saturate at 11 mol/kg. This behavior suggests a gradual destruction in the hydrogen-bond network until the saturation concentration. The surprisingly large concentration required for the saturation supports an interpretation in which the ions affect the structure of water only within their first hydration shell. The study is complemented by density-functional-theory calculations and molecular dynamics simulations.

Effect of the hydrophobic alcohol chain length on the hydrogen-bond network of water.

The microscopic structure of the hydrogen-bond network of water-alcohol mixtures was studied using X-ray Raman scattering (XRS). To systematically examine how the hydrogen-bond network of water is affected by an increasing size of the hydrophobic group, small linear alcohols (methanol, ethanol, and propanol) in constant mole fractions were studied. The oxygen K-edge spectra were not altered upon hydration of the alcohols beyond a simple superposition of signals from alcohol and water. The experiment thus indicates that alcohols do not have a substantial effect on the structure of the hydrogen-bond network of water. In particular, no apparent breaking or forming of the hydrogen bonds is observed to take place in the overall structure. In addition, there is no indication of changes in the tetrahedrality of the hydrogen-bond network of water in the vicinity of alcohol molecules.

Phase separation and nanocrystal formation in GeO

The temperature-induced phase separation (disproportionation) and Ge nanocrystal formation in bulk amorphous germanium monoxide (a-GeOx,x≈1) are studied both in situ and ex situ by measurements of the x-ray absorption near edge structure at the Ge K-edge and x-ray diffraction. The considerable amount of suboxides contained in the native a-GeO samples decreases with increasing annealing temperature. The phase separation sets in at a temperature of 260±20 °C and is almost completed at a temperature of 450±18 °C before nanocrystal formation occurs. Ge nanocrystals of a few nanometers in diameter are observed for an annealing temperature of 509±15 °C. The time dependence of the phase separation and the effect of different annealing procedures are discussed. The presented results provide important information for the production of Ge nanocrystals embedded in amorphous oxide matrices which are relevant for optoelectronic applications.

Direct tomography imaging for inelastic X-ray scattering experiments at high pressure

A method to separate the non-resonant inelastic X-ray scattering signal of a micro-metric sample contained inside a diamond anvil cell (DAC) from the signal originating from the high-pressure sample environment is described. Especially for high-pressure experiments, the parasitic signal originating from the diamond anvils, the gasket and/or the pressure medium can easily obscure the sample signal or even render the experiment impossible. Another severe complication for high-pressure non-resonant inelastic X-ray measurements, such as X-ray Raman scattering spectroscopy, can be the proximity of the desired sample edge energy to an absorption edge energy of elements constituting the DAC. It is shown that recording the scattered signal in a spatially resolved manner allows these problems to be overcome by separating the sample signal from the spurious scattering of the DAC without constraints on the solid angle of detection. Furthermore, simple machine learning algorithms facilitate finding the corresponding detector pixels that record the sample signal. The outlined experimental technique and data analysis approach are demonstrated by presenting spectra of the Si L2,3-edge and O K-edge of compressed α-quartz. The spectra are of unprecedented quality and both the O K-edge and the Si L2,3-edge clearly show the existence of a pressure-induced phase transition between 10 and 24 GPa.

Protonation Dynamics and Hydrogen Bonding in Aqueous Sulfuric Acid.

Hydration of sulfuric acid plays a key role in new-particle formation in the atmosphere. It has been recently proposed that proton dynamics is crucial in the stabilization of these clusters. One key question is how water molecules mediate proton transfer from sulfuric acid, and hence how the deprotonation state of the acid molecule behaves as a function concentration. We address the proton transfer in aqueous sulfuric acid with O K edge and S L edge core-excitation spectra recorded using inelastic X-ray scattering and with ab initio molecular dynamics simulations in the concentration range of 0-18.0 M. Throughout this range, we quantify the acid-water interaction with atomic resolution. Our simulations show that the number of donated hydrogen bonds per Owater increases from 1.9 to 2.5 when concentration increases from 0 to 18.0 M, in agreement with a rapid disappearance of the pre-edge feature in the O K edge spectrum. The simulations also suggest that for 1.5 M sulfuric acid SO4(2-) is most abundant and that its concentration falls monotonously with increasing concentration. Moreover, the fraction of HSO4(-) peaks at ∼12 M.

Temperature dependence of crystal field excitations in CuO

We report a study on the temperature dependence of charge-neutral crystal field (dd) excitations in cupric oxide, using nonresonant inelastic x-ray scattering spectroscopy. Thanks to a very high-energy resolution (ΔE = 60 meV), we observe thermal effects on the dd excitation spectrum fine structure between temperatures of 10-320 K. The spectra broaden considerably with increasing temperature, consistently with an enhancement of the coupling between crystal field excitations and the temperature-dependent continuum of states above the band gap. We discuss this and other mechanisms that may explain this temperature dependence.

Sulphur Kβ emission spectra reveal protonation states of aqueous sulfuric acid

In this paper we report an X-ray emission study of bulk aqueous sulfuric acid. Throughout the range of molarities from 1 M to 18 M the sulfur Kβ emission spectra from H2SO4 (aq) depend on the molar fractions and related deprotonation of H2SO4. We compare the experimental results with results from emission spectrum calculations based on atomic structures of single molecules and structures from ab initio molecular dynamics simulations. We show that the S Kβ emission spectrum is a sensitive probe of the protonation state of the acid molecules. Using non-negative matrix factorization we are able to extract the fractions of different protonation states in the spectra, and the results are in good agreement with the simulation for the higher part of the concentration range.

In situ characterization of the decomposition behavior of Mg(BH4)2 by X-ray Raman scattering spectroscopy

We present an in situ study of the thermal decomposition of Mg(BH4)2 in a hydrogen atmosphere of up to 4 bar and up to 500 °C using X-ray Raman scattering spectroscopy at the boron K-edge and the magnesium L2,3-edges. The combination of the fingerprinting analysis of both edges yields detailed quantitative information on the reaction products during decomposition, an issue of crucial importance in determining whether Mg(BH4)2 can be used as a next-generation hydrogen storage material. This work reveals the formation of reaction intermediate(s) at 300 °C, accompanied by a significant hydrogen release without the occurrence of stable boron compounds such as amorphous boron or MgB12H12. At temperatures between 300 °C and 400 °C, further hydrogen release proceeds via the formation of higher boranes and crystalline MgH2. Above 400 °C, decomposition into the constituting elements takes place. Therefore, at moderate temperatures, Mg(BH4)2 is shown to be a promising high-density hydrogen storage material with great potential for reversible energy storage applications.

High-resolution nonresonant x-ray Raman scattering study on rare earth phosphate nanoparticles

We report high-resolution x-ray Raman scattering studies of high-order multipole spectra of rare earth excitations (the absorption edge) in nanoparticles of the phosphates LaPO4, CePO4, PrPO4, and NdPO4. We also present corresponding data for La excitations (the edge) in LaPO4. The results are compared with those from calculations by atomic multiplet theory and for the dipole contribution to the La transition from a calculation using time-dependent density functional theory (TDLDA). Agreement with the atomic multiplet calculations for the high-order multiplet spectra is remarkable in the case of the spectra. In contrast, we find that the shallow semicore excitations in LaPO4 manifest a relatively broad band and an apparent quenching of spin-orbit splitting. The more sophisticated TDLDA, which has earlier been found to explain dipolar spectra well in Ba compounds, is less satisfactory here in the case of La.