Diffraction contrast from small voids as observed by electron microscopy

An investigation is made of the contrast in transmission electron microscopy at cavities or gas filled voids which are in equilibrium in a solid matrix. The dynamic theory of electron diffraction is used in the two beam approximation and normal as well as anomalous absorption is taken into account, it is shown that the presence of a cavity can be adequately represented by a phase angle α = 2 Π s z₂, which depends on the thickness z₂ of the void and on the diffraction conditions (through the excitation error S). The main result is, that in contrast with other treatments, the image depends on the vertical position of the void. The theory accounts for the large variety of images which are observed under different diffraction conditions, for different void sizes and depths.