Most electron diffraction is performed with high energy electrons whose wavelengths
are orders of magnitude smaller than the interplanar spacings
in most crystals. For example, for 100 keV electrons l
< 3.7 x 10-12 m.
Typical lattice parameters for crystals are around 0.3 nm.
Electrons are charged, light particles and their penetration
into solids is very limited.LEED
are therefore considered to be surface science techniques,
while transmission electron diffraction is limited to specimens
less than 1 mm thick. Transmission electron diffraction
is usually carried out in a transmission electron microscope
A typical electron diffraction
pattern for a crystalline specimen is shown here.
Features of electron diffraction
There are three particularly important features of diffraction using high energy
(1) Since l is very small, Bragg angles are
also small, so the Bragg Law can be simplified to:
- l = 2dqB
|Electron energy / keV
||Wavelength / pm
(2) The diameter of the Ewald sphere is very large compared to the
size of the unit cell in the reciprocal lattice.
(3) Lenses are able to focus the diffraction pattern and to change the
camera length, which is equivalent to moving the film in an x-ray experiment.
Lenses can also be used to change from Fraunhofer to Fresnel