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The periodic potential in crystals causes the amplitude of the accelerating electron (e.g. 200 keV in TEM) to be transferred backandforth between the transmitted and diffracted wavefunctions. This transfer process can be explained by dynamical theory. The crystal has a periodic potential that is weak compared with an electron energy of 200 keV. At the Laue condition ( s= 0), the physical distance between two backandforth transfers is called the “extinction distance.”
Assuming the TEM specimen is a perfect crystal, in two beam condition the intensity (I_{g}) of diffracted electron beam can be given by (based on dynamical theory),
 [4134a]
 [4134b]
 [4134c]
where,
s_{eff}  Effective deviation parameter
ξ_{g}  Extinction distance
V  Volume of the unit cell
λ  Electron wavelength
Γ_{g}  Structure factor of the unit cell for diffraction g
s  Deviation parameter
t  Crystal thickness
Equation 4134a for dynamical theory is valid when the TEM sample is thick and s is about zero, but is not valid in kinematical theory. From Equation 4134c we can know that the extinction distance decreases with increase of scattering (increase of Γ_{g}). When s is equal to zero (exact Bragg condition) we have s_{eff} = 1/ξ_{g}, meaning the transmitted (I_{0}) and diffracted (I_{g}) intensities has a periodicity of ξ_{g} in TEM specimen depth indicated in Figure 4134 in two beam condition. Here, I_{0} = 1  I_{g} in two beam condition. Equation 4134a also indicates that the intensity of these low order diffraction disks
fluctuates with thickness.
Figure 4134. The diffracted intensity (I_{g}) showing a periodicity of ξ_{g} in TEM
specimen depth in two beam condition. I_{g} is the intensity of transmitted beam.
