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.

Experimental Station 13-3

Beam line 13-3 utilizes a spherical grating monochromator on an elliptical polarized undulator (EPU) operating in the photon energy range from 230 eV to 1700 eV with full polarization control (linear vertical/horizontal and circular left/right) covering the C, O, F K-edges, 3d transition metal L-edges, and 4f rare-earth element M-edges.

Experimental Station 10-1

Beam line 10-1 is a wiggler side-station beam line for soft x-ray core-level spectroscopy. The endstation is equipped with a hemispherical photoelectron analyzer (SES-100), and detectors for Total Fluorescence Yield (TFY; Diode), Total Electron Yield (TEY; Channeltron) and Partial Electron Yield (PEY). It has a generic chamber that is compatible with both ex-situ and in-situ characterization of a large number of samples simultaneously. XAS can be recorded in AEY, TEY, FY yields.

Experimental Station 8-2

Beam line 8-2 can be used to probe a wide range of core levels using photoemission and x-ray absorption spectroscopies. At BL 8-2, two end-stations are available for the general user program. The first end-station is a high throughput setup for soft x-ray spectroscopy measurement – upgrades in progress include a fixed position multipurpose chamber with a load lock system for high throughput sample loading, capable of probing a wide range of materials with absorption and photoemission techniques.

Experimental Station 14-3a

Beam line 14-3a, located on the upstream table of the hutch of the BL14-3 bending magnet side station, is dedicated to bulk x-ray absorption spectroscopy of biological, materials, and geological samples in the tender x-ray photon energy range 2.1-5.0 keV. BL14-3 is the only beam line at SSRL capable of obtaining spectroscopy data at the phosphorous K edge. In this configuration the beam is unfocused over a size of 1 mm x 6 mm to allow for high energy-resolution measurements on homogenous samples.

Experimental Station 6-2b

Beam line 6-2b is a wiggler end-station dedicated to High Resolution Hard X-ray Spectroscopy. The end station combines three multicrystal Johann spectrometers that enable X-ray Emission Spectroscopy (XES), Resonant Inelastic X-ray Scattering (RIXS), High-Energy Resolution Fluorescence Detected X-ray Absorption Spectroscopy (HERFD-XAS) and X-ray Raman Spectroscopy (XRS) techniques.

Mass-selected Nanoparticles of PtxY as Model Catalysts for Oxygen Electroreduction

July 31, 2014

A team of researchers from the Technical University of Denmark and  the SUNCAT Institute at the SLAC National Accelerator Laboratory and Stanford University has demonstrated the superior performance of nanoparticles of platinum-yttrium (PtxY) as catalysts for oxygen electroreduction.

Correlation of Lithium-Ion Battery Performance with Structural and Chemical Transformations

April 30, 2014

Rechargeable lithium-ion batteries are widely used in applications ranging from consumer electronics to electric vehicles. An important feature of a high-quality battery is a long lifetime, i.e. a large number of possible charge-discharge cycles. However, every cycle introduces changes in the battery’s electrode material, limiting its cyclability. A research collaboration has recently examined the occurring structural and chemical changes in the electrode material during cycling and linked them to the performance of lithium-ion batteries.

 

Hydrogen Adsorption Induces Interlayer Carbon Bond Formation in Supported Few-Layer Graphene

February 28, 2014

Graphite and diamond are two distinct forms of the same element, carbon. Nevertheless, their properties could not be any more different. For instance, diamond is extremely hard and can be used in cutting tools. Graphite, on the other hand, is soft and used in pencils. Graphite can be converted into diamond in a process that usually requires very high pressure. However, scientists have recently suggested an alternative route to obtain diamond-like structures from graphite – at least on the nanoscale.

Revealing the Nature of Emergent Ferromagnetism at an Oxide Heterointerface

September 30, 2013

Perovskites are mineral oxides with unique properties of great interest to scientists. Many of these materials show remarkable transitions in their behavior. The perovskites lanthanum aluminium oxide (LAO) and strontium titanium oxide (STO), for instance, are insulators. However, when sandwiched together to an LAO/STO heterostructure, the material can conduct electricity at its interface. Researchers can tune conductivity and other emergent properties by doping the perovskites and hope to exploit heterostructures in future industrial applications such as new electronic devices.

Nanoparticulate FeS as an Effective Redox Buffer to Prevent Uraninite (UO2) Oxidation

August 31, 2013

Uranium (U) is one of the most prevalent radionuclide contaminants in soils and groundwater across the world as a result of nuclear fuel production, weapons manufacturing, and research activities. The environmental risks posed by U are determined largely by the degree of its mobility, which strongly depends on redox conditions.  Under oxic conditions, U(VI) is soluble and forms stable complexes with carbonate and calcium in groundwater.

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