X-ray Absorption Spectroscopy

XAS is a core-level spectroscopy technique, using a photo-excited electron from a core level (e.g. 1s or 2p) to probe unoccupied valence levels as well as the neighboring atomic structure. The ionization of core levels requires photons in the energy in the X-ray range, and spectroscopy requires an intensive continuous energy-spectrum, hence XAS is carried out at synchrotron radiation sources that provide both.

The measurement is conducted by scanning the incident photon energy using a monochromator. Once a sufficient energy is reached to ionize the atom at its core level, the absorption steeply increases at what is known as an absorption edge. Every element in the periodic table has a unique absorption edge, making the technique conveniently element-specific. The portion of the spectrum around the edge, known as the X-ray absorption near-edge structure (XANES) is a rich probe for the electronic structure of the unoccupied states as the low-energy photoelectron occupies these states. Chemical information about the oxidation state and local geometry is obtained from the XANES. As the incident energy is increased, more energy is transferred to the photoelectron, exciting it to the continuum of states and enabling it to back-scatter from neighboring atoms within ca. 10 Å. The back-scattering of the photo-electron causes a quantum-mechanical overlap between its initial and final state, causing an oscillatory modulation of absorption, or the extended X-ray absorption fine-structure (EXAFS). The Fourier-transform of the EXAFS is a radial distribution function, from which bond distances, number and speciation of neighboring atoms can be extracted.

XAS is the core technique of our group, since it is powerful in observing the chemical state and atomic structure in catalysts, especially under reaction conditions.

Compositions of Stardust Impact Tracks and Terminal Particles in Aerogel by Hard X-ray Microprobe at SSRL

December 19, 2006

An international collaboration that included researchers at SSRL has used x-ray scanning microprobe fluorescence techniques at BL6-2 to characterize the elemental chemistry of samples from comet 81P/Wild-2 brought back aboard the Stardust spacecraft last January. Twenty-three aerogel samples containing cometary particles were analyzed by the 175-member Preliminary Examination Team, and five of those samples were studied at SSRL. This collaboration provided the first look at the Stardust samples after the return, and results are presented in several publications in the December 15 issue of Science.

Reexamination of Lead(II) Coordination Preferences in Sulfur-Rich Sites:Implications for a Critical Mechanism of Lead Poisoning

December 21, 2005

Research performed at SSRL has provided insight into why lead is so damaging to the healthy development of young children. Scientists from the University of Michigan and Northwestern University used x-ray absorption spectroscopy at SSRL to understand how lead can interfere with proteins that help transform DNA blueprints into working proteins that run the body.

Damage by X-rays: A Case Study for Metallo-Protein Crystallography

September 30, 2005

X-rays intended to elucidate the structure of biomolecules may actually damage and alter key parts of the molecules. A research team led by a group from Lawrence Berkeley National Laboratory (in collaboration with researchers from Max-Planck-Institut Mülheim, ESRF, SSRL, and TU Berlin and Freie Universität, Berlin) discovered this while investigating the Mn4Ca complex, a site crucial for splitting water into oxygen during photosynthesis.

Characterization of a Genuine S=1/2 Fe(V) Complex

June 30, 2005

Iron metals oxidize to rust, losing electrons and gaining positive charge. Iron metals typically exist in an oxidation state of +2 or +3 (2 or 3 electrons less than a neutral iron atom). However, chemists have long thought that iron compounds with even higher oxidation states play important roles in enabling chemical reactions in metal-containing proteins.

The First Cadmium Enzyme - Carbonic Anhydrase 2 from the Marine Diatom Thalassiosira weissflogii

May 31, 2005

Cadmium is known to be extremely toxic to mammals, and is generally viewed alongside mercury as an environmental problem and toxic element that is not used by nature in any way. A Brief Communication in the May 5 issue of the journal Nature shows that we need to revise our opinion of cadmium. The paper reports the purification and characterization of a previously unknown metalloenzyme from the marine diatom Thalassiosira weissflogii that specifically uses cadmium to achieve its biological function. This is the first cadmium enzyme that has been discovered.

Catalyst Design: X-rays Cross-examine the Fuel Cell Volcano Plot

June 22, 2012

Changing the electronic structure of a metal in order to “tune” its affinity to catalytic reaction intermediates is a key element in catalyst design. Tailor-made catalysts with a carefully adjusted ratio of two or more different alloy components are particularly needed in fuel cells, which could efficiently power electric vehicles – without the range limitations of current batteries.

The Structure of the First Coordination Shell in Liquid Water

April 30, 2004

In ice, each water molecule is surrounding by 4 other molecules in a tetrahedral arrangement (left). The new result on liquid water shows that the molecules are connected only with 2 others. This implies that most molecules are arranged in strongly hydrogen bonded rings (middle) or chains (right) embedded in a disordered clusternetwork connected mainly by weak hydrogen bonds. The oxygen atoms are red and the hydrogen atoms grey in the water (H2O) molecules.

Remediation of Uranium-contaminated Ground Water at Fry Canyon, Utah

November 28, 2003

A new technology that acts like a giant underground filter is successfully beginning to clean up the uranium contaminating an aquifer in a remote Utah canyon. Uranium contamination in groundwater is a serious problem because the toxic metal can travel long distances in underground aquifers, which are vital sources of fresh water for people, animals and agriculture. Recent research at SSRL showed that the filters-called PRBs (permeable reactive barrier) do intercept uranium, but in an unexpected way that has important implications for monitoring, costs, and future technology selection.

X-ray Absorption Spectroscopy Catches the Chemical Form of Mercury in Fish

August 29, 2003

The presence of "methyl mercury" in fish is well-known, but until now the detailed chemical identity of the mercury has remained a mystery. In an x-ray absorption spectroscopy study published in the August 29 issue of Science (Science 301, 2003: 1203;Science now: Murky Picture on Fish Mercury), SSRL scientists report that the chemical form of mercury involves a sulfur atom (most likely in a so-called aliphatic form). The study presents significant new knowledge - because the toxic properties of mercury (or any element) are critically dependent upon its chemical form - and represents an important milestone in developing an understanding of how harmful mercury in fish might actually be. The study was carried out by SSRL staff scientists Ingrid Pickering and Graham George and postdoctoral fellow Hugh Harris using SSRL's structural molecular biology beam line 9-3. The very high flux, excellent beam stability and state-of-the-art detector technology allowed the team to measure samples of fish containing micromolar levels of mercury, much lower than had previously been possible.

Fate and Stability of Cr Following Reduction by Microbially Generated Fe(II)

May 30, 2003

Industrial activities have led to widespread chromium (Cr) contamination in the environment. Although Cr is an essential element for humans, the hexavalent form is toxic, mutagenic and carcinogenic. Consequently, the presence of Cr in the environment poses a serious threat to human and animal welfare. However, the toxicity of Cr is a function of oxidation state. For example, hexavalent Cr has a high solubility in soils and groundwater and, as a consequence, tends to be mobile in the environment.

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