3. Using anomalous diffusion

The interaction of X-rays with a given atom j is modeled by a scattering factor f _j which characterizes the scattering of the atom, taking that of a free electron as the unit. Under many conditions, f _j is a real number, i.e. there is no phase difference between the wave scattered by the atom and that scattered by a free electron (Thompson scattering). In reality, the electrons of an atom are not free, but are distributed in orbitals of given energies, and the atom presents absorption discontinuities corresponding to the excitation of deep electronic layers. The approximations of classical theory are no longer justified if the incident X-ray energy is close to the atom's absorption threshold. A resonance phenomenon is triggered, resulting in a phase change of...