EELS can be applied with several
techniques, but always involves the bombardment
of a sample with a monoenergetic beam of electrons.
The technique is frequently used in association
with Transmission Electron Microscopy (TEM) or
Scanning TEM (STEM). The electrons impinging on
the sample may lose energy by a variety of mechanisms.
These losses can reveal the composition of the
sample in TEM or STEM. Plasmon losses are a frequent
cause of energy loss. Plasmons are collective
excitations of the electron gas in the material
and are typically several electron Volts in magnitude.
Phonon losses can also occur, which are much smaller,
and the energy spread of the monoenergetic beam
must be particularly small to detect such losses.
Phonons are quantized sound waves within the solid.
In TEM or STEM, the losses predominantly
occur in the bulk of the sample, as the beam travels
through the thin specimen to the EELS detector
the other side. In surface science techniques,
the electron beam is usually reflected of the
surface resulting in a sharp peak corresponding
to elastically scattered electrons with a number
of peaks at lower energy which correspond to plasmon
or other excitations.
By examining energy losses at high
resolution (about 30 meV), as in HREELS, data
concerning the vibrations of molecules on surfaces
can be determined. Hence HREELS provides complementary
information to RAIRS.
Another technique that is worthy
of mention is spin polarised EELS or SPEELS which can provide information on phenomena such
as magnetic coupling and exchange excitation processes.