X-ray absorption spectroscopy
Over the last two decades, X-ray absorption spectroscopy (XAS) has emerged as an incisive probe of the local structure around selected atomic species in solids and liquids. Foremost among its strengths are its applicability to amorphous materials and its tunability; that is, the ability to probe the environments of different elements in the sample by selecting the incident X-ray energy. Outside of single crystal X-ray diffraction and nuclear magnetic resonance (NMR), few other techniques allow such probing of molecular structure.
XAS Applications
- EXAFS
- Coordination environment
- Bond lengths
- Local disorder
- Valence state
- XANES
- Fermi energy
- Local coordination geometry
Extended X-ray absorption fine structure (EXAFS)
When a monochromatic X-ray beam is directed through a sample, and as the energy of the X-rays is gradually increased such that it crosses an absorption edge of one of the elements-of-interest (EOI) in the sample, the transmitted X-ray light will contain small variations in absorbance, on the high energy side of the absorption edge, that provide information about the structural environment of the atoms surrounding the element whose absorption edge is being examined.
In more detail, X-ray absorption is dominated by photoelectron absorption where the photon is completely absorbed—creating a photoelectron and hole pair. The kinetic energy of the excited photoelectron is equal to the difference between the exciting photon and the electron's binding energy. To a first approximation, the final energy state of the photoelectron is modified by a single scattering by each of the surrounding atoms. From a quantum mechanical viewpoint, the photoelectron is treated as a wave whose wavelength (λ) is described by the de Broglie relation (λ= h/p), where p is the momentum of the photoelectron and h is Planck's constant. For the EXAFS experiment, the momentum may be determined by the free electron relation: p2/2m = hν - Eo, where the X-ray photon of frequency ν has an energy hν, Eo is the binding energy and m is the mass of the photoelectron.
For an isolated atom, the photoelectron can be represented as an outgoing wave. The surrounding atoms will scatter the outgoing wave. The final state is the superposition of the outgoing and scattered waves. The total amplitude of the electron wave function will thus be enhanced or reduced—thus modifying the probability of absorption of the X-ray beam. In this way, the variation of the fine structure in EXAFS is a direct consequence of the wave nature of the photoelectron. Variations in the observed phase with wavelength of the photoelectron then depends on the distance of the excited atom to that of the backscattering atoms. Variation of the backscattering strength, as a function of the energy of the photoelectron, depends on the atomic number of the backscattering atoms.
X-ray absorption near-edge structure (XANES)
The X-ray absorption spectrum can be divided into near edge and extended fine
structure. The X-ray absorption near-edge structure (XANES) is extended in the
first 30-40 eV past the absorption edge, while the extended X-ray absorption
fine structure (EXAFS) covers the photon energy range from about 40 eV to about
1000 eV past the edge. The interpretation of XANES spectra is substantially more
complicated than EXAFS spectra.
XANES is associates with the excitation process of a core electron to bound and
quasi-bound states, where the bound states interacting with the continuum are
located below the ionization threshold (vacuum level) and the quasi-bound states
interacting with the continuum are located above or near the threshold. Like
EXAFS, XANES contains information about the electronic state of the X-ray
absorbing atom and the local surrounding structure. However, unlike EXAFS, since
the excitation process essentailly involves multi-electron and multiple
scattering interactions, interpretation of XANES spectra is substantially more
complicated.
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