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Electron

  Electron Diffraction     1 of 8
 

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 and RHEED 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 (TEM).

Diffraction Patterns

A typical electron diffraction pattern for a crystalline specimen is shown here.

Electron diffraction pattern

Features of electron diffraction

There are three particularly important features of diffraction using high energy electrons:

(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 Diffracting planes qB
50 5.355 Cu111 0.75
100 3.701 Al200 0.54
300 1.969 Si200 0.32

(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.

Questions imageWhat electron lens strength is required to focus the diffraction pattern at the fixed screen position shown?


Lenses can also be used to change from Fraunhofer to Fresnel diffraction conditions.

 

 
 

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