X-ray absorption near edge structure

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X-ray absorption near edge structure (XANES), also known as near-edge X-ray absorption fine structure (NEXAFS), is a simple way to study how materials absorb X-rays very close to the energy needed to remove a core electron. It gives important clues about what elements are there, their oxidation state, how they’re bonded, and their immediate surroundings.

How it works (in plain terms)
- When X-rays hit a material, they can kick a core electron into an unoccupied state. This creates a short-lived excited state with a “core hole.”
- The way the material absorbs X-rays in this near-edge region depends on the final states that the excited electron can reach. These final states are influenced by the local environment around the absorbing atom.
- Because the final states can involve interactions with nearby atoms, XANES captures details about the local structure and the electronic state of the absorbing element.

XANES, NEXAFS, and EXAFS: what the names mean
- XANES (the “near edge” part) focuses on the region very close to the absorption edge and is sensitive to oxidation state and local geometry.
- NEXAFS is another name for the same idea, often used for surfaces and molecules studied with soft X-rays.
- EXAFS (Extended X-ray Absorption Fine Structure) looks further away from the edge and mainly provides distances to neighboring atoms and the arrangement around the absorber. In short, XANES/NEXAFS cover the near edge; EXAFS covers the outer region.

Energy ranges in simple terms
- XANES/NEXAFS region: roughly a few to a few dozen electronvolts above the edge. It’s dominated by resonances and multiple scattering, giving fingerprints of the local environment.
- EXAFS region: from about 150 eV above the edge up to a few thousand eV, where single or multiple scattering by neighboring atoms reveals interatomic distances.
- There’s also a low-energy/early part (NEXAFS) where backscattering by nearby atoms is strong, and a high-energy part (EXAFS) where backscattering is simpler and mainly informs about distances.

Experimental notes
- Soft X-ray measurements require powerful, tunable light from a synchrotron and are often done by detecting emitted photons (fluorescent yield) or by measuring the current in the sample (total electron yield).
- Soft X-rays are absorbed by air, so experiments are done with the X-ray beam in a vacuum or helium environment.
- NEXAFS can probe buried layers and very small amounts of material because it sums over many possible final states, not just the direct signal from a single emitted particle.

Theory in the simplest terms
- The process creates a short-lived core-hole state and an excited electron. The spectrum’s peaks come from how that excited electron interacts with its surroundings (the nearby atoms and their electrons).
- Near the edge, multiple scattering and many-body effects shape the spectrum, making XANES/NEXAFS rich in information about local structure and chemical state.
- At higher energies, EXAFS examines how the emitted electron is scattered by neighboring atoms, giving bond lengths and coordination.

Why scientists use XANES/NEXAFS
- Elemental specificity: it can pick out signals from one element even in a mix.
- Chemical state and coordination: helps identify oxidation state, whether an atom sits in, say, an octahedral or tetrahedral site, and subtle structural distortions.
- Buried layers and surfaces: because it collects all final states, it can detect signals from thin surface layers or layers beneath a surface.
- Unoccupied electronic structure: it provides clues about the states that electrons can move into above the Fermi level.

Plain-language summary
XANES/NEXAFS is a versatile, element-specific tool that tells you about the local world around a particular atom in a material. By watching how X-ray absorption changes very close to the absorption edge, scientists learn about oxidation state, bonding, and geometry. By going farther from the edge (EXAFS), they measure distances to neighboring atoms. The technique relies on soft X-rays from a synchrotron, often detects either emitted photons or electrons, and is especially powerful for studying tiny amounts of material, buried layers, and surface chemistry.

A few historical notes
- The term XANES was coined in 1980 to describe sharp absorption peaks from resonances near the edge.
- The term NEXAFS appeared around 1983, and is used interchangeably with XANES in many contexts, especially for surfaces and molecules.

Common uses
- Fingerprinting the chemical state and local structure in mixtures.
- Studying oxidation states in complex samples (for example, trace elements in soils or minerals).
- Investigating the unoccupied electronic structure and how atoms are bonded in a material.

In short, XANES/NEXAFS is a near-edge X-ray absorption technique that reveals detailed, element-specific information about the local electronic structure and geometry around absorbing atoms, making it a powerful tool across chemistry, geology, environmental science, and materials research.