Thomas Huthwelker

A high-repetition rate scheme for synchrotron-based picosecond laser pump/x-ray probe experiments on chemical and biological systems in solution

We present the extension of time-resolved optical pump/x-ray absorption spectroscopy (XAS) probe experiments towards data collection at MHz repetition rates. The use of a high-power picosecond laser operating at an integer fraction of the repetition rate of the storage ring allows exploitation of up to two orders of magnitude more x-ray photons than in previous schemes based on the use of kHz lasers. Consequently, we demonstrate an order of magnitude increase in the signal-to-noise of time-resolved XAS of molecular systems in solution. This makes it possible to investigate highly dilute samples at concentrations approaching physiological conditions for biological systems. The simplicity and compactness of the scheme allows for straightforward implementation at any synchrotron beamline and for a wide range of x-ray probe techniques, such as time-resolved diffraction or x-ray emission studies.

A new endstation at the Swiss Light Source for ultraviolet photoelectron spectroscopy, X-ray photoelectron spectroscopy, and X-ray absorption spectroscopy measurements of liquid solutions.

A new liquid microjet endstation designed for ultraviolet (UPS) and X-ray (XPS) photoelectron, and partial electron yield X-ray absorption (XAS) spectroscopies at the Swiss Light Source is presented. The new endstation, which is based on a Scienta HiPP-2 R4000 electron spectrometer, is the first liquid microjet endstation capable of operating in vacuum and in ambient pressures up to the equilibrium vapor pressure of liquid water at room temperature. In addition, the Scienta HiPP-2 R4000 energy analyzer of this new endstation allows for XPS measurements up to 7000 eV electron kinetic energy that will enable electronic structure measurements of bulk solutions and buried interfaces from liquid microjet samples. The endstation is designed to operate at the soft X-ray SIM beamline and at the tender X-ray Phoenix beamline. The endstation can also be operated using a Scienta 5 K ultraviolet helium lamp for dedicated UPS measurements at the vapor-liquid interface using either He I or He II α lines. The design concept, first results from UPS, soft X-ray XPS, and partial electron yield XAS measurements, and an outlook to the potential of this endstation are presented.

Kinetics of the heterogeneous reaction of nitric acid with mineral dust particles: an aerosol flowtube study

The heterogeneous reaction of HNO3 with mineral dust aerosol (Arizona Test Dust) was studied in an aerosol flow tube at atmospherically relevant conditions (298 K, approximately 1 atm, 6-60% RH) and using radioactively labelled HNO3. The uptake of nitric acid was found to depend on HNO3 and H2O concentrations in the gas phase. A reaction mechanism is suggested to describe the heterogeneous interaction, involving Langmuir type adsorption and surface reaction. This mechanism is incorporated in a flux based aerosol kinetic model framework that is able to reproduce the observations within the range of experimental conditions. The experiments show that the reactive surface sites of the relatively calcium poor, but silicate rich dust surface are efficiently depleted at higher HNO3 concentrations in the gas phase or longer exposure times. A set of kinetic parameters is extracted from the data, which can be used to calculate uptake coefficients as function of time, concentration and humidity for use in atmospheric chemistry models to improve especially the representation of the effects of relative humidity on dust aging and to allow following dust aging with time.

Interaction of gaseous elemental mercury with snow surfaces: laboratory investigation

The interaction of elemental mercury with ice surfaces based on the migration behaviour at sub-ppt concentrations in a packed bed flow tube is discussed. Analysis shows that elemental mercury interacts only weakly with ice surfaces and suggests an adsorption enthalpy of −28 ± 2k J mol −1 . The experiments further reveal an adsorption equilibrium constant, which quantifies the partitioning of elemental mercury between the surface and the gas phase, with a value of 12 cm at 140 K. Extrapolation to environmental conditions gives a range for the equilibrium constant of 3 × 10 −4 to 4 × 10 −3 cm at 230 K. S Supplementary data are available from stacks.iop.org/ERL/3/045009

The Molecular Environment of Phosphorus in Sewage Sludge Ash: Implications for Bioavailability

Producing a P fertilizer from sewage sludge ash (SSA) is a strategy to efficiently recycle P from a secondary raw material. The P speciation in four SSAs was characterized before and after the removal of heavy metals by a thermo-chemical treatment that involved CaCl addition. We chose complementary techniques to determine the direct P speciation, including X-ray powder diffraction, solid-state P direct-polarization magic-angle spinning nuclear magnetic resonance, and X-ray absorption near edge structure. Results from these techniques were compared with operational and functional speciation information obtained from a sequential P extraction and a plant biotest with Italian ryegrass grown on a soil-sand mixture with little available P. The speciation of P in untreated and thermo-chemically treated SSAs depended on their elemental composition. At a molar ratio of Ca:P ≤ 2, SSAs contained combinations of polymorphs of AlPO, β-tricalcium phosphate, and apatite-like P species. In SSAs with a molar ratio of Ca:P > 2, an apatite-like molecular environment was predominant. The thermo-chemical process induced an increase in crystalline phases and enhanced the crystallinity of the P species. The structural order of the bulk sample was the most decisive parameter in controlling the P availability of the studied SSAs to plants. We conclude that, to produce a high-quality fertilizer and despite of the successful heavy metal removal, the thermo-chemical process requires further development toward enhanced P bioavailability.

Microstructure characteristics during hydrate formation and dissociation revealed by X-ray tomographic microscopy

AbstractDespite much progress over the past years in fundamental gas hydrate research, frontiers to the unknown are the early beginning and early decomposition of gas hydrates in their natural, submarine environment: gas bubbles meeting ocean water and forming hydrate, and gas starting to escape from the surface of a hydrate grain. In this paper we report on both of these topics, and present three-dimensional microstructure results obtained by synchrotron radiation X-ray cryo-tomographic microscopy (SRXCTM). Hydrates can precipitate when hydrate-forming molecules such as methane exceed solubility, and combine with water within the gas hydrate stability zone. Here we show hydrate formation on surfaces of bubbles from different gas mixtures and seawater, based on underwater robotic in situ experiments in the deep Monterey Canyon, offshore California. Hydrate begins to form from the surrounding water on the bubble surfaces, and subsequently grows inward into the bubble, evidenced by distinct edges. Over time, the bubbles become smaller while gas is being incorporated into newly formed hydrate. In contrast, current understanding has been that hydrate decomposition starts on the outer surface of hydrate aggregates and grains. It is shown that in an early stage of decomposition, newly found tube structures connect well-preserved gas hydrate patches to areas that are dissociating, demonstrating how dissociating areas in a hydrate grain are linked through hydrate that is still intact and will likely decompose at a later stage. FigureThe boundaries of a gas hydrate grain: excepting for the matrix (transparent, not shown), one can see tubular structures, pores from decomposition, and bubbles.

Introducing Time Resolution to Detect Ce3+ Catalytically Active Sites at the Pt/CeO2 Interface through Ambient Pressure X-ray Photoelectron Spectroscopy

X-ray photoelectron spectroscopy has been employed for the qualitative and quantitative characterization of both model and real catalytic surfaces. Recent progress in the detection of photoelectrons has enabled the acquisition of spectra at pressures up to a few tens of millibars. Although reducing the pressure gap represents a remarkable advantage for catalysis, active sites may be short-lived or hidden in the majority of spectator species. Time-resolved experiments, conducted under transient conditions, are a suitable strategy for discriminating between active sites and spectators. In the present work, we characterized the surface of a Pt/CeO2 powder catalyst at 1.0 mbar of a reacting mixture of carbon monoxide and oxygen and, by means of time resolution, identified short-lived active species. We replaced oxygen with nitrogen in the reaction mixture while fast-detecting the core level peaks of cerium. The results indicate that active Ce3+ sites form transiently at the surface when the oxygen is switched off. Analysis of the depth profile shows that Ce3+ ions are located at the ceria surface. The same experiment, performed on platinum-free ceria, reveals negligible reduction, indicating that platinum boosts the formation of Ce3+ active sites at the interface.

Ion fractionation in young sea ice from Kongsfjorden, Svalbard

Abstract The fractionation of major sea-water ions, or deviation in their relative concentrations from Standard Mean Ocean Water ratios, has been frequently observed in sea ice. It is generally thought to be associated with precipitation of solid salts at certain eutectic temperatures. the variability found in bulk sea-ice samples indicates that the fractionation of ions depends on the often unknown thermal history of sea ice, which affects the structure of pore networks and fate of solid salts within them. Here we investigate the distribution of ions in Arctic sea ice that is a few weeks old with a reconstructible thermal history. We separate the centrifugable (interconnected) and entrapped (likely disconnected) contributions to the ice salinity and determine their ion fractionation signatures. the results indicate that differential diffusion of ions, rather than eutectic precipitation of cryohydrates, has led to significant ion fractionation. the finding emphasizes the role of coupled diffusive–convective salt transport through complex pore networks in shaping the biogeochemistry of sea ice.

Supersaturated calcium carbonate solutions are classical

Ions and ion pairs are the species that lead to CaCO3 nucleation. Mechanisms of CaCO3 nucleation from solutions that depend on multistage pathways and the existence of species far more complex than simple ions or ion pairs have recently been proposed. Herein, we provide a tightly coupled theoretical and experimental study on the pathways that precede the initial stages of CaCO3 nucleation. Starting from molecular simulations, we succeed in correctly predicting bulk thermodynamic quantities and experimental data, including equilibrium constants, titration curves, and detailed x-ray absorption spectra taken from the supersaturated CaCO3 solutions. The picture that emerges is in complete agreement with classical views of cluster populations in which ions and ion pairs dominate, with the concomitant free energy landscapes following classical nucleation theory.

Towards hybrid pixel detectors for energy-dispersive or soft X-ray photon science

A novel hybrid pixel detector is evaluated and its potential for low-noise/low-energy detection and energy-dispersive photon science is highlighted.