What is twobeam condition How to use it to determine the Bur

What is two-beam condition? How to use it to determine the Burger\'s vector of a dislocation?

Solution

Two-beam conditions are used for imaging of dislocations and other extended crystal defects by diffraction contrast in a TEM i.e., the direct beam is diffracted by one lattice plane only with reciprocal lattice vector g. The complex amplitudes of the direct beam T and the diffracted beam S can be calculated according to the two-beam dynamical diffraction theory of Howie and Whelan eq.

For real crystals, diffraction contrast is determined by the strain field of the extended crystal defect, i.e., the displacement function (z) yields local changes of the excitation error s and thereby local changes of the amplitudes of the direct beam and the diffracted beam.

It is shown that the usual method of Burgers\' vector analysis in the Transmission Electron Microscope (TEM) using invisibility criteria is unlikely to succeed for ice. A method is outlined using a novel application of computer-simulated imaging which can be used to perform such an analysis.