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.

Bacterial Sulfur Storage Globules

January 31, 2002

Sulfur is essential for all life, but it plays a particularly central role in the metabolism of many anaerobic microorganisms. Prominent among these are the sulfide-oxidizing bacteria that oxidize sulfide (S2-) to sulfate (SO42-). Many of these organisms can store elemental sulfur (S0) in "globules" for use when food is in short supply (Fig. 1). The chemical nature of the sulfur in these globules has been an enigma since they were first described as far back as 1887 (1); all known forms (or allotropes) of elemental sulfur are solid at room temperature, but globule sulfur has been described as "liquid", and it apparently has a low density – 1.3 compared to 2.1 for the common yellow allotrope α-sulfur.

Intercation of Toxic Metals with Complex BioFilm/Mineral/Solution Interfaces

November 30, 2001

Sorption reactions on particle surfaces can dramatically affect the speciation, cycling and bioavailability of essential micronutrients (i.e. PO43-, Cu, Zn etc.) and toxic metals and metalloids (i.e. Pb, Hg, Se, As) in soils and aquatic environments. Considerable attention has been focused on understanding metal sorption reactions at a molecular/mechanistic level and the effects of metal concentration, pH, ionic strength, and complexing ligands on the ways in which metal ions bind to the surfaces of common mineral phases such as Fe-, Mn- and Al-(hydr)oxides and clays. However, a significant fraction of mineral surfaces in natural environments are extensively colonized by microbial organisms, which can also be potent sorbents for metals due to the large number of reactive functional groups that decorate the cell walls and outer membranes of bacterial surfaces. 

Experimental Station 14-3b

Beam line 14-3a, located on the downstream table in the BL14-3 hutch, is a bending magnet side station dedicated to x-ray imaging and micro x-ray absorption spectroscopy of biological, biomedical, materials, and geological samples.  Most often used for data collection at the S K edge, BL14-3 is the only SSRL beam line capable of XAS at the P K edge. During imaging mode, a Kirkpatrick-Baez (KB) mirror system is used to achieve microfocus with a beam size of ~ 5 x 5 microns. BL14-3b is equipped with a Vortex silicon drift detector and ionization chambers for imaging.

Experimental Station 11-2

Beam line 11-2 is a high-flux XAS station dedicated to molecular biogeochemical and interface sciences. It is optimized for challenging XAS measurements on dilute or radioactive samples and interfaces. To support these experiments, BL11-2 is equipped with collimating and focusing optics, a "double double" Si(220) LN2-cooled monochromator, and a 100-pixel monolithic solid state Ge detector array.

Experimental Station 10-2a

Beam line 10-2 is a wiggler end-station that splits time between the front hutch (BL10-2a), which is instrumented for x-ray absorption spectroscopy imaging, and the rear hutch (BL10-2b), which has a dedicated 6-circle diffractometer that is used for materials scattering.  The front hutch utilizes a polycapillary focusing optic to provide a beam spot as small as ~10 microns up to 200 microns.   Samples as large as 300 x 600 mm can be measured in one image, but larger samples can also be accommodated.  In addition, BL10-2 can be set up for confocal x-ray fluorescence using two polycapillary f

Experimental Station 9-3

Beam line 9-3 is a wiggler side-station dedicated to biological x-ray absorption spectroscopy and EXAFS measurements of dilute solutions and single crystals. BL9-3 is equipped with a 100-element Ge monolithic solid-state detector in addition to ionization chambers and PIPS/Lytle detectors. A dedicated LHe cryostat allows for routine low temperature measurements on solutions. A Huber-Kappa goniometer, a LHe cryostream, a focusing polycapillary and a MAR CCD detector are available (in addition to specialized software) for single crystal measurements.

Experimental Station 7-3

Beam line 7-3 is a wiggler side-station dedicated for x-ray absorption spectroscopy and EXAFS measurements on dilute biological systems. BL7-3 is equipped with a 30-element Ge solid-state detector in addition to ionization chambers and Lytle/PIPS detectors. A dedicated LHe cryostat allows for routine low temperature measurements. The unfocused beam on BL7-3 is ideal for samples especially susceptible to radiation damage, such as high-valent intermediates and other oxidized species.

Experimental Station 4-3

Beam line 4-3 is a wiggler side-station dedicated for x-ray absorption spectroscopy and EXAFS measurements on biological, environmental, catalysis, and materials systems. This station enables tender x-ray measurement (S K-edge and up, i.e. 2.4-5 keV) in addition to hard x-rays ( up to 11 keV).  It is setup with a He flight path from the beam line optics to the sample. The beam is collimated and unfocused to allow for high energy-resolution measurements on homogenous samples.

Experimental Station 4-1

Beam line 4-1 is a high-flux station optimized for x-ray absorption spectroscopy and EXAFS experiments requiring x-rays with energies between ~ 6 and 38 keV.  This energy range includes most of the transition metals (all rows), lanthanides and actinides, P-block elements, alkaline and alkaline earths.

Experimental Station 2-2

Beam line 2-2 is a bending magnet side-station beam line for x-ray absorption spectroscopy, Quick-EXAFS, and EXAFS measurements of samples with absorption edges in the energy range 4.5 - 37 keV. BL2-2 is equipped with a water-cooled double crystal monochromator, with multiple crystal sets available [Si(111), Si(220)]. Available detectors include ionization chambers, Lytle, 13-element Ge array, and Vortex detectors. Specialized facilities for in-situ catalysis research include gas handling systems and mass flow control.  The beam line sees limited use for white beam experiments.

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