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.

Biotic-Abiotic Pathways: A New Paradigm for Uranium Reduction in Sediments

March 31, 2013

As part of a larger, DOE-funded investigation into bioremediation of uranium in contaminated aquifers, a group of SSRL scientists made a surprising discovery about how uranium ions behave in the environment. In addition to overturning current scientific models, this research will lead to more efficient, less costly methods for uranium cleanup and mining.

The Chemistry of Bromine in Terrestrial and Marine Environments

November 30, 2012

Recent work at SSRL has helped reveal a previously unrecognized wealth of bromine chemistry in the environment, where bromine in seawater has long been thought to exist as inorganic bromide, while bromides in soil were considered so unreactive that they've routinely been used as a hydrological tracer.

The reality bromine chemistry in the environment is much more complex. X-ray absorption spectroscopic (XAS) studies conducted by Leri, et al. at SSRL Beam Lines 2-3 and 4-3, as well as at the ALS and NSLS, reveal a complicated association between bromine and organic carbon in both sea water and soil.

A New Slant on a Cellular Balancing Act — the Copper-sensing Repressor of Mycobacterium tuberculosis

January 31, 2007

Scientists have discovered a gene for a protein that regulates the cellular response to copper in the bacterium that causes tuberculosis. These findings, reported in the January issue of Nature Chemical Biology, explain how a wide variety of bacteria control copper concentrations within their cells, and this understanding could lead to new treatments for tuberculosis.

Delocalized Molecular Orbitals of the [6Fe6S] Cluster of the FeFe-Hydrogenase

February 28, 2007

The FeFe-hydrogenases are of great interest because they can catalyze both the forward and reversed dihydrogen uptake/evolution reactions. Under optimal conditions a single molecule of FeFe-hydrogenase can produce approximately 9000 molecules of hydrogen per second. This translates into a theoretical capacity for refueling the hydrogen tank of the Space Shuttle within 30 minutes. Thus, hydrogenases are considered as desirable biological targets for hydrogen-based energy production and utilization technologies.

Structural Insights into FeMo Cofactor Biosynthesis

February 27, 2006

SSRL and Stanford scientists, in collaboration with a team from UC Irvine, have gotten the first look into how the metal active center of an enzyme that is largely responsible for fertilizing plants is assembled. This enzyme, which is called nitrogenase, certain bacteria employ to turn nitrogen from the air into a form that plants can use for healthy growth. In contrast to the enzymatic reaction, manufacturing nitrogen fertilizer chemically requires extreme pressures and temperatures and thus huge amounts of energy.

Holey Germanium - New Routes to Ordered Nanoporous Semiconductors

July 25, 2006

Porous nanoscale materials often have useful properties because of their proportionally large surface areas. Now, UCLA scientists have devised a way to make porous germanium, a semiconductor used in fiber optics and electrical components. This discovery means that nanoporous materials could soon be used to develop new kinds of solar cells or highly sensitive electronic sensors.

An Octahedral Coordination Complex of Iron(VI)

July 25, 2006

Chemists have synthesized and characterized a new, highly reactive form of iron that promises to deepen our understanding of this important element. Iron is found in abundance in the natural world, and in its ionized form plays a crucial role in virtually all living processes.

A Fern Fatale - X-ray Absorption Spectroscopy Imaging of an Arsenic-Loving Fern

September 25, 2006

The toxicity of arsenic is widely known, but perhaps less widely appreciated is that it's the level of toxicity critically depends on the chemical form. The fern Pteris vittata, is one of a small group of plants that actively accumulates to a startling degree - an arsenic hyperaccumuatlor. P. vittata absorbs arsenic from soil, typically present as the relatively benign arsenate, and changes its chemical form to arsenite, which is one of the more toxic kinds of arsenic. The plant thrives on this toxic regimen, and it most likely does this to defends itself from hungry herbivores. The ability of P. vittata to take up arsenic has generated much excitement because of potential applications for environmental cleanup of drinking water and of contaminated sites.

Structural Sequestration of Uranium in Bacteriogenic Manganese Oxides

October 30, 2006

Uranium contamination is a major concern at Department of Energy sites and decommissioned mining and ore processing facilities around the U.S. Migration of uranium has contaminated ground water in several locations, and the threat remains for further contamination unless costly measures are taken to isolate the contaminates and stop their spread.

Where Water is Oxidized to Dioxygen: Structure of the Photosynthetic Mn4Ca Cluster

November 30, 2006

Billions of years ago, primitive bacteria developed a way to harness sunlight to split water molecules into protons, electrons and oxygen-the cornerstone of photosynthesis. Now, a team of scientists has taken a major step toward understanding this process by deriving the precise structure of the catalytic metal-cluster center containing four manganese atoms and one calcium atom (Mn4Ca) that drives this water-splitting reaction. This catalytic center resides in a large protein complex, called photosystem II, found in plants, green algae, and cyanobacteria. The international team was led by scientists from LBNL, and includes scientists from Germany's Technical and Free Universities in Berlin, the Max Planck Institute in Mülheim, and from SSRL.


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