X-ray absorption spectroscopy

XASEdges

X-ray Absorption Spectroscopy (XAS) is a widely used technique for determining the local geometric and/or electronic structure of matter. The method is applicable to virtually all types of materials, including gases, liquids, and solids, and can be used to study a wide variety of atoms. The technique hinges on the measurement of the absorption of X-rays as their energy is varied across a range that includes the binding energy of core electrons of the atoms in the sample. This absorption can provide detailed information about the structure and electronic state of the atoms within a material.

Principles of X-ray Absorption Spectroscopy

X-ray Absorption Spectroscopy is based on the principle that when X-ray photons have enough energy to dislodge tightly bound core electrons from atoms, the absorption coefficient of the material experiences a sharp increase. This phenomenon is known as the X-ray Absorption Edge. The energy at which this increase occurs is specific to each element, providing a direct method for elemental identification. Beyond the edge, the absorption spectrum can be divided into two main regions: the X-ray Absorption Near Edge Structure (XANES) and the Extended X-ray Absorption Fine Structure (EXAFS).

XANES

The XANES region, located just above the absorption edge, provides information about the valence state, coordination number, and symmetry of the absorbing atom. This region is sensitive to the electronic structure of the atom and can thus be used to study the chemical bonding and oxidation state of the elements in the sample.

EXAFS

The EXAFS region extends from the edge of the XANES out to several hundred electron volts above the absorption edge. The oscillations in this region are due to the interference between the outgoing photoelectron wave and the waves backscattered from surrounding atoms. By analyzing these oscillations, one can extract detailed information about the distances, coordination numbers, and disorder of the atoms surrounding the absorber.

Applications of X-ray Absorption Spectroscopy

X-ray Absorption Spectroscopy has found applications in a wide range of fields, including chemistry, physics, materials science, and biology. It is particularly useful for studying materials under in-situ conditions, such as catalysts during chemical reactions, materials under high pressure or temperature, and biological samples in their natural hydrated state. XAS can also be used to study the electronic structure of materials, including band structure and density of states.

Experimental Setup

X-ray Absorption Spectroscopy experiments are typically performed at synchrotron radiation facilities, which provide the intense and tunable X-ray beams required for the technique. The sample is irradiated with X-rays, and the intensity of the transmitted beam is measured as a function of energy. By comparing the absorption spectrum of the sample to that of a reference material, quantitative information about the sample can be extracted.

Data Analysis

The analysis of XAS data involves several steps, including background subtraction, normalization, and fitting of the XANES and EXAFS regions to theoretical models. This analysis can be complex, requiring sophisticated software and a good understanding of the physical processes involved.

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