Pseudo-weak-phase-object approximation in high-resolution electron microscopy. I. Theory

On the basis of the dynamical electron diffraction theory of Cowley & Moodie [Acta Cryst. (1957), 10, 609-619], a new formula has been derived for the multiple-beam image intensity expressed in terms of the projected potential distribution as well as the specimen thickness. According to this formula, crystals for which the weak-phase-object approximation does not hold can be treated by the pseudo-weak-phase-object approximation (PWPOA) up to a certain critical thickness. Here the real crystal is replaced by its imaginary isomorph, where the constituent heavy atoms behave as lighter atoms than those of the real crystal, and vice versa. The validity of the PWPOA is discussed and confirmed by the comparison of the images of chlorinated Cu phthalocyanine calculated with PWPOA and the multislice method.