Section 4.3.1.5. Absorption effects

Any scattering process, whether elastic or inelastic, which removes energy from the set of diffracted beams being considered, may be said to constitute an absorption process. For example, for a measurement of the intensities of the elastically scattered, sharp Bragg reflections from a crystal, any process which gives diffuse background scattering or results in a detectable loss of energy gives rise to absorption.

The diffracted amplitudes in such cases may be calculated, at least as a first approximation, in terms of a complex potential, , containing an imaginary part due to an `absorption function' and a small added real part . Then under the crystallographic sign convention, . Correspondingly, for a centrosymmetric crystal, the structure amplitude becomes complex and may be written Under the appropriate conditions of observation, important contributions to the imaginary and real additions to the structure amplitudes may be given by the excitation of phonons, plasmons, or electron transitions, or by diffuse scattering due to crystal defects or disorder.

The additional terms and , however, are not invariant properties of the crystal structure but depend on the conditions of the diffraction experiment, such as the accelerating voltage and orientation of the incident beam, the aperture or resolution of the recording system, and the use of energy filtering or discrimination. In spite of this, it may often be convenient to treat them as being produced by phenomenological complex potentials, defined for a limited range of experimental conditions.