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Electron energy loss spectroscopy (EELS) is a family of techniques that measure the change in kinetic energy of electrons after they interact with a specimen. This technique is used to determine the atomic structure and chemical properties of a specimen, including the type and quantity of atoms present, chemical state of atoms and the collective interactions of atoms with their neighbors. Some of these techniques include spectroscopy, energy-filtered transmission electron microscopy (EFTEM), and DualEELS™.
As electrons pass through a specimen, they interact with atoms of the solid. Many of the electrons pass through the thin sample without losing energy. A fraction will undergo inelastic scattering and lose energy as they interact with the specimen. This leaves the sample in an excited state. The material can de-excite by giving up energy typically in the form of visible photons, x-rays or Auger electrons.
As the incident electron interacts with the sample, it changes both its energy and momentum. You can detect this scattered incident electron in the spectrometer as it gives rise to the electron energy loss signal. The sample electron (or collective excitation) carries away this additional energy and momentum.
Core-loss excitations occur when tightly bound core electrons are promoted to a higher energy state by the incident electron. The core electron can only be promoted to an energy that is an empty state in the material. These empty sates can be bound states in the material above the Fermi level (so-called anti-bonding orbitals in the molecular orbital picture). The states can also be free-electron states above the vacuum level. It is the sudden turn-on of the scattering at the Fermi energy and the probing of empty states which makes the EELS signal sensitive to both the atom type and its electronic state.
You can visualize the initial spectral features in the core-loss excitations when you align the Fermi level with the zero-loss peak (ZLP) of the spectrum. The edges can now be seen as the point where the electrons lose enough energy to promote the core level atomic electrons to the Fermi level. This analogy fails to reproduce the scattering above the Fermi level but is helpful to visualize the core level edge sudden increase in intensity.
A typical energy loss spectrum includes several regions. The first peak, the most intense for a very thin specimen, occurs at 0 eV loss (equal to the primary beam energy) and is therefore called the zero-loss peak. It represents electrons that did not undergo inelastic scattering but may have been scattered elastically or with an energy loss too small to measure. The width of the zero-loss peak mainly reflects the energy distribution of the electron source. It is typically 0.2 – 2.0 eV but may be as narrow as 10 meV or lower in a monochromated electron source.
For more information on the EELS family of techniques, please visit EELS.info, an educational site.
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