Ralf Thomann
Feb.2002
center: Elastic Imaging: Imaging of elastically scattered electrons only - contrast enhancement
right: Inelastic Imaging : Imaging of the inelastic scattered electrons - elementspecific imaging, ESI
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Ralf Thomann
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When passing through a specimen, electrons interact with it. Some are scattered without losing energy (elastic scattering, e.g. in the case of crystals Bragg scattering) which implies that the electrons are deflected by a certain scattering angle and no longer form part of the transmitted beam. Other electrons lose energy inside the specimen by interacting for example with electrons in the specimen itself. Often this takes the form of a beam electron ejecting an electron from its orbit around the nucleus of an atom in the specimen. The beam electron loses an amount of energy (at the very least the amount needed to eject the specimen electron) . When the energy-loss electrons are separated according to their energy, they form an energy-loss spectrum.
The spectra have a characteristic feature of a fall off in intensity. Superimposed on these spectra are elemental characteristic peaks that correspond to the absorption edges for the elements that interact with the electron. These spectra are complementary to EDX spectra in that they are of higher energy resolution (1-2eV) and more sensitive to light elements. However, these spectra are more difficult to quantify. The spectral energy resolution is ~ 0.5eV.
With the use of an energy selecting slit, a real-time chemical map (image) of a specific region of the energy loss spectra can be acquired on a Slow-Scan camera. These images reveal quantitative chemical maps. In addition to the chemical imaging, the energy-selecting slit can be used in reverse. By simply selecting only the zero loss region, thus eliminating the inelastic scattered electrons, images through thicker specimens are obtained. |
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center: Elastic Imaging: Imaging of elastically scattered electrons only - contrast enhancement right: Inelastic Imaging : Imaging of the inelastic scattered electrons - elementspecific imaging, ESI |
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