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X-ray absorption fine structure (XAFS) spectroscopy

Schematic term diagram of Fe. The binding energy of the 1s electron of an Fe atom is 7112 eV. The main K-edge is caused by transitions from 1s core to continuum-like states (A). The lowest unfilled bound states are the 3d states. Hence, the energetically
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X-ray absorption fine structure (XAFS) spectroscopy is the measurement of the mass absorption coefficient of a material as a function of excitation energy with high energy resolution. With this technique it is possible to investigate unoccupied electronic states.

Two kinds of analysis have to be distinguished: X-ray absorption near edge spectroscopy (XANES or near edge X-ray absorption fine structure (NEXAFS)) and Extended X-ray absorption fine structure (EXAFS).

Depending on the sample two modes of the experimental setup are common: In transmission mode one measures the fraction of the incoming beam and the transmitted beam as a function of energy. In the second mode, the fluorescent mode, the intensity of a fluorescent line of interest as a function of the excitation energy is measured.

XANES/NEXAFS

© Bernhard Hesse
Fe K-XANES spectra of a pure Fe sample and a sample with Fe in mineral form
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The near edge region is in the energy range from about 10 eV before the main edge to some tens of eV above the main edge.

Chemical information can be extracted from XANES measurements. Physically a jump in cross-section, as it is shown in the figure above, corresponds to the transition from a core orbital to an unoccupied bound state. The energies of the unfilled orbitals depend on the environment of the observed atom.

The main edge in a K-XANES spectrum is caused by the transition of the 1s electron to continuum-like states. The structure shortly above the edge to higher energies is caused also by transitions to continuum-like states and by complicated multiple scattering processes, while the pre-edge is caused by transitions of the 1s electron into unfilled bound states.

EXAFS

With EXAFS the main interest is in the extended spectrum out to much higher exciting energies.

Above the main K-edge the liberated photoelectron can propagate from the source atom as a spherical wave. This outgoing wave can be backscattered by neighboured atoms. This process becomes evident at several tens of eV above the main edge. The interference of the outgoing and backscattered wave are causing the EXAFS oscillations. Analysis of the EXAFS signal provides information about the chemical coordination environment of the observed atom.

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