What is two-beam and multiple-beam condition in TEM and its application?
please response based on Bragg's law .
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Diffraction according to Bragg's law occurs whenever the Ewald sphere (sphere of radius: 1/wavelength) touches a point on the reciprocal lattice. In many/multi-beam conditions, diffraction occurs for all the reciprocal lattice points (diffraction directions) according to Weiss zone law and lying CLOSE to the Ewald sphere. However, the intensity of those points are not as intense as the central [000] beam since the former points do not lie EXACTLY on the Ewald sphere.
For two-beam conditions, the reciprocal lattice is rotated by appropriate double tilt (X and Y) so that a particular g(hkl) satisfying the Weiss zone law is brought EXACTLY on the Ewald sphere. This makes that particular diffraction spot as intense as the central beam. It is known as two-beam since only the [000] and [hkl] (i.e. only two directions) are on the Ewald sphere. As a result of this tilt, the other directions satisfying the Weiss zone law moves even farther away from the Ewald sphere resulting in significant decrease in their intensities.
You may interpret it as the diffraction directions in any particular zone axis, which are not in the two-beam condition transfer their intensity to the direction in the said condition.
Diffraction according to Bragg's law occurs whenever the Ewald sphere (sphere of radius: 1/wavelength) touches a point on the reciprocal lattice. In many/multi-beam conditions, diffraction occurs for all the reciprocal lattice points (diffraction directions) according to Weiss zone law and lying CLOSE to the Ewald sphere. However, the intensity of those points are not as intense as the central [000] beam since the former points do not lie EXACTLY on the Ewald sphere.
For two-beam conditions, the reciprocal lattice is rotated by appropriate double tilt (X and Y) so that a particular g(hkl) satisfying the Weiss zone law is brought EXACTLY on the Ewald sphere. This makes that particular diffraction spot as intense as the central beam. It is known as two-beam since only the [000] and [hkl] (i.e. only two directions) are on the Ewald sphere. As a result of this tilt, the other directions satisfying the Weiss zone law moves even farther away from the Ewald sphere resulting in significant decrease in their intensities.
You may interpret it as the diffraction directions in any particular zone axis, which are not in the two-beam condition transfer their intensity to the direction in the said condition.
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