Eels is the electron energy loss spectrum. Electron Energy Loss Spectrum (EELS)? It is a series of techniques to measure the change of kinetic energy after the interaction between electrons and samples. This technique is used to determine the atomic structure and chemical characteristics of samples, including the types and quantities of elements, the chemical state of elements and the collective interaction between elements and adjacent atoms. Some technologies include spectroscopy, energy filtered transmission electron microscopy (EFTEM) and DualEELS.

When electrons pass through the sample, they will interact with atoms in the solid. Many electrons pass through a thin sample without losing energy. Some of them will be inelastic scattering and lose energy when interacting with atoms. This will make the sample in an excited state. By analyzing the energy that usually exists in the form of visible photons, X-rays or Auger electrons, the material can be deactivated.

When the incident electrons interact with the sample, both energy and momentum will change. You can detect this scattered incident electron in the spectrometer because it will signal the loss of electron energy. Sample electrons (or collective excitation) will take away extra energy and momentum.

When tightly bound core electrons are excited to high energy state by incident electrons, core loss excitation will occur. Core electrons can only be excited to the empty energy in the material. These empty states can be bound states above Fermi level in materials (so-called antibonding orbitals in molecular orbital diagrams). This state can also be a free electron state higher than the vacuum level. The sudden opening of Fermi level scattering and the detection of empty state make EELS signal sensitive to atomic type and electronic state.

By aligning Fermi level with spectral zero loss peak (ZLP), the initial spectral characteristics in core loss excitation can be revealed. The edge can be regarded as the point where the electron energy loss is enough to make the atomic electrons at the core level reach Fermi level. This simulation can not reproduce the scattering above Fermi level, but it is helpful to visualize the sudden increase of the edge intensity of the core energy level.

A typical energy loss spectrum includes multiple regions. The first peak, that is, the position where the intensity of the extremely thin sample is the highest, appears at the loss of 0 eV (equal to the initial beam energy), so it is called zero loss peak. It represents electrons that are not inelastic scattered, but may be elastically scattered or the energy loss is too small to be measured. The width of zero loss peak mainly reflects the energy distribution of the electron source. The width is usually 0.2–2.0 eV, but it may be as narrow as 10 MeV or less in a monochromatic electron source.