The dynamical theory of diffraction describes the interaction of
waves with a regular
lattice. The wave fields traditionally described are
X-rays,
neutrons or
electrons and the regular lattice are atomic
crystal structures or
nanometer-scale
multi-layers or self-arranged systems. In a wider sense, similar treatment is related to the interaction of light with optical
band-gap materials or related wave problems in
acoustics. The sections below deal with dynamical diffraction of X-rays.
Principle
The dynamical theory of diffraction considers the wave field in the periodic potential of the crystal and takes into account all multiple scattering effects. Unlike the
kinematic theory of diffraction which describes the approximate position of
Bragg or
Laue diffraction peaks in
reciprocal space, dynamical theory corrects for refraction, shape and width of the peaks, extinction and interference effects. Graphical representations are described in
dispersion surfaces around reciprocal lattice points which fulfill the boundary conditions at the crystal interface.
Outcomes
The crystal potential by itself leads to
refraction and
specular reflection of the waves at the interface to the crystal and delivers the
refractive index off the Bragg reflection. It also corrects for refraction at the Bragg condition and combined Bragg and specular reflection in grazing incidence geometries.
A Bragg reflection is the splitting of the dispersion surface at the border of the
Brillouin zone in reciprocal space. There is a gap between the dispersion surfaces in which no travelling waves are allowed. For a non-absorbing crystal, the reflection curve shows a range of
total reflection, the so-called
Darwin plateau. Regarding the quantum mechanical
energy of the system, this leads to the
band gap structure which is commonly well known for electrons.
Upon Laue diffraction, intensity is shuffled from the forward diffracted beam into the Bragg diffracted beam until extinction. The diffracted beam itself fulfills the Bragg condition and shuffles intensity back into the primary direction. This round-trip period is called the Pendellösung period.
The extinction length is related to the Pendellösung period. Even if a crystal is infinitely thick, only the crystal volume within the extinction length contributes considerably to the diffraction in
Bragg geometry.
Anomalous absorption effects take place due to a
standing wave patterns of two wave fields. Absorption is stronger if the standing wave has its anti-nodes on the lattice planes, i.e. where the absorbing atoms are, and weaker, if the anti-nodes are shifted between the planes. The standing wave shifts from one condition to the other on each side of the
Darwin plateau which gives the latter an asymmetric shape.
J. Als-Nielsen, D. McMorrow: Elements of Modern X-ray physics. Wiley, 2001 (chapter 5: diffraction by perfect crystals).
André Authier: Dynamical theory of X-ray diffraction. IUCr monographs on crystallography, no. 11. Oxford University Press (1st edition 2001/ 2nd edition 2003).
ISBN0-19-852892-2.
R. W. James: The Optical Principles of the Diffraction of X-rays. Bell., 1948.
M. von Laue: Röntgenstrahlinterferenzen. Akademische Verlagsanstalt, 1960 (German).
Z. G. Pinsker: Dynamical Scattering of X-Rays in Crystals. Springer, 1978.
B. E. Warren: X-ray diffraction. Addison-Wesley, 1969 (chapter 14: perfect crystal theory).
W. H. Zachariasen: Theory of X-ray Diffraction in Crystals. Wiley, 1945.
Boris W. Batterman, Henderson Cole: Dynamical Diffraction of X Rays by Perfect Crystals. Reviews of Modern Physics, Vol. 36, No. 3, 681-717, July 1964.
H. Rauch, D. Petrascheck, “Grundlagen für ein Laue-Neutroneninterferometer Teil 1: Dynamische Beugung”, AIAU 74405b, Atominstitut der Österreichischen Universitäten, (1976)
H. Rauch, D. Petrascheck, “Dynamical neutron diffraction and its application” in “Neutron Diffraction”, H. Dachs, Editor. (1978), Springer-Verlag: Berlin Heidelberg New York. p. 303.
K.-D. Liss: "Strukturelle Charakterisierung und Optimierung der Beugungseigenschaften von Si(1-x)Ge(x) Gradientenkristallen, die aus der Gasphase gezogen wurden", Dissertation, Rheinisch Westfälische Technische Hochschule Aachen, (27 October 1994),
urn:nbn:de:hbz:82-opus-2227