Resonance effects in low and high energy electron diffraction by crystals

The relation between an anomalous intensity effect in patterns from high energy electron diffraction by crystals, and the so-called resonance effect in LEED, is discussed mainly on the basis of experimental observations with electrons in both energy ranges, using crystals of zincblende, magnesium oxide and gallium arsenide. It is concluded that these two effects are essentially of the same nature. It is pointed out, that the intensity enhancement of the specular spot and other Laue-zone spots, which occurs under conditions of resonance, is particularly conspicuous when the specular spot is also a Bragg reflection spot. The nature of the wavefield generated in the crystal at resonance is discussed in connexion with other phenomena which appear under the resonance condition, such as the overall intensity enhancement of electron scattering, the increase of secondary electron emission and the variation of X-ray emission yield.