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In
electron energy
loss spectroscopy (EELS) a material is exposed to a beam of electrons with a known, narrow range of kinetic
energies. Some of the electrons
will undergo inelastic scattering, which means that they lose energy and have their paths slightly and randomly
deflected. The amount of energy
loss can be measured via an electron
spectrometer and interpreted in terms of what caused the energy loss. Inelastic interactions include phonon excitations, inter and intra band transitions,
plasmon excitations, inner shell ionizations, and ?Erenkov radiation. The
inner-shell ionizations are particularly useful for detecting the elemental
components of a material. For example, one might find that a
larger-than-expected number of electrons
comes through the material with 285 ev (electron
volts, a unit of energy)
less energy
than they had when they entered the material. It so happens that this is
about the amount of energy
needed to remove an inner-shell electron
from a carbon
atom. This can be taken as evidence that there's a
significant amount of carbon
in the part of the material that's being hit by the electron
beam. With some care, and looking at a wide range of energy losses, one can determine the types of atoms, and the numbers of atoms of
each type, being struck by the beam. The scattering angle (that is, the
amount that the electron's
path is deflected) can also be measured, giving information about the
dispersion relation of whatever material excitation caused the inelastic
scattering.
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