Abstract
RECENTLY, Cowley and Rees1 reported some interesting work on the fine structure of electron diffraction patterns of crystals of magnesium oxide, cadmium oxide, etc. (multiple rings or spots) by application of high-resolution electron diffraction cameras. They confirmed the view of Sturkey and Frevel, who suggested that this fine structure arises from refraction at the faces of the regularly shaped crystal cubes. If the normals of the faces by which the beam of electrons enters and leaves the crystal lie in the same plane with the beam and the normal of the net plane2, one can derive an expression relating the angular displacement of the beam ε to the angles φ1 and φ2 (angle between face normal and incident and refracted electrons respectively):(V is inner potential, E is accelerating potential, θ0 Bragg angle for V = 0). This deviation having values of 10−4 radians (V = 10 volts, E = 50,000 volts, φ = 45°) or 3 x 10−3 cm. at a specimen-plate distance of 30 cm., high resolution will be necessary to detect it.
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Cowley and Rees, Proc. Phys. Soc., 59, 288 (1947). Nature 158, 550 (1946). See also Sturkey and Frevel, Phys. Rev., 68, 56 (1945) and Hillier and Baker, Phys. Rev., 68, 98 (1945).
If this condition is not fulfilled one has to take a more general formula derived by Cowley and Rees (ref. 1).
Raether, H., Métaux et Corrosion (Jan. 1947).
Kranert, W., Leise, K. H., and Raether, H., Z. Phys., 122, 248 (1944). In this paper the formula given for θ was θ =, which had been derived by putting approximately θ + θ0 = 2θ0: developed, it gives the exact solution 2. They are equivalent in the regions applied for the evaluation of electrolytically polished surfaces.
For example, Heidenreich, R. D., and Sturkey, J. App. Phys., 16, 97 (1945), Fig. 1. Kinder, E., Naturwiss, 81, 149 (1943). This effect must not be confused with the parallel strips of strongly inclined crystals the origin of which explains the dynamical theory, nor with the Fresnel diffraction strings in the out-of-focus image of a crystal edge.
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RAETHER, H. Refraction Effects in Electron Diffraction and in Electron Microscopy. Nature 161, 311 (1948). https://doi.org/10.1038/161311a0
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DOI: https://doi.org/10.1038/161311a0
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