A SIMPLE INTUITIVE THEORY FOR ELECTRON-DIFFRACTION

It is shown that the dynamical diffraction can simply be described in real space using the property that electrons are trapped in the electr ostatic potential of the atomic columns. Due to this channelling effec t, the electron diffraction can be highly dynamical inside each column , and at the same time retain a one-to-one relationship with the cryst al structure. This description does not require the crystal to be peri odic. Influence of adjacent columns can be treated using a perturbatio n theory. If the crystal is sufficiently thin, i.e. of the order of 10 nm, and the accelerating voltage is not too high (e.g. 100-300 keV), the motion of the electrons is almost perfectly periodic with depth. T he theory shows how the depth periodicity is related to the mass/thick ness of the column which allows the exit wavefunction to be parametriz ed in a simple analytical form. These results open perspectives to sol ve the inverse problem of how to derive the projected structure of the object from the exit wavefunction.