X-ray absorption spectroscopy

Key point: X-ray absorption spectra can be used to determine the oxidation state of an element in a compound and to investigate its local environment.

As mentioned in the previous section, the intense X-ray radiation from synchrotron sources may be used to eject electrons from the cores of elements present in a compound. X-ray absorption spectra (XAS) are obtained by varying the photon energy across a range of energies at which electrons in the various atoms present in a compound can be excited and ionized (typically between 0.1 and 100 keV). The characteristic absorption energies correspond to the binding energies of different inner-shell electrons of the various elements present. Thus the frequency of an X-ray beam may be swept across an absorption edge of a selected element and information on the oxidation state and neighbourhood of this chosen chemical element obtained. Figure 8.33 shows a typical X-ray absorption spectrum. Each region of the spectrum can provide different useful information on the chemical environment of the element under investigation:

1. Just prior to the absorption edge is the ‘pre-edge’ where core electrons are excited to higher empty orbitals but not ejected. This ‘pre-edge structure’ can provide information on the energies of excited electronic states and also on the local symmetry of the atom.

2. In the edge region, where the photon energy, E is between Ei and Ei+10 eV, where Ei is the ionization energy, the ‘X-ray absorption near-edge structure’ (XANES) is observed. Information that can be extracted from the XANES region includes oxidation state and the coordination environment, including any subtle geometrical distortions. The near-edge structure can also be used as a ‘fingerprint’, as it is characteristic of a specific environment and valence state. The presence and quantity of a compound in a mixture may be determined from analysis of this region of the spectrum.

3. The ‘near-edge X-ray absorption fine structure’ (NEXAFS) region lies between E_i+10 eV and E_i+50 eV. It has particular application to chemisorbed molecules on surfaces because it is possible to infer information about the orientation of the adsorbed molecule.

4. The ‘extended X-ray absorption fine structure’ (EXAFS) region lies at energies greater than E_i+50 eV. The photoelectrons ejected from a particular atom by absorption of X-ray photons with energies in this region may be backscattered by any adjacent atoms. This effect can result in an interference pattern that is detected as periodic variations in intensity at energies just above the absorption edge. In EXAFS these variations are analysed to reveal the nature (in terms of their electron density) and number of nearby atoms and the distance between the absorbing atom and the scattering atom. An advantage of this method is that it can provide bond lengths in amorphous samples and for species in solutions.

Figure 8.33 A typical X-ray absorption edge spectrum defining the various regions discussed in the text.

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قسم الشؤون الفكرية يعلن مشاركة ممثلين من 18 دولة إفريقية في خدمة زائري الإمام العسكري (عليه السلام)

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مواضيع ذات صلة

Thermal analysis

X-ray fluorescence elemental analysis

CHN analysis

Atomic absorption spectroscopy

Ionization and detection methods

Photoelectron spectroscopy

Spectroscopic monitoring of titrations and kinetics

Absorption spectroscopy

Neutron diffractionNeutron diffraction

Single-crystal X-ray diffraction

Powder X-ray diffraction

X-ray diffraction