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Optimal Modified Two‐Beam Representation of Dynamical X‐Ray Diffraction near a Three‐Beam Point
Author(s) -
Juretschke H. J.
Publication year - 1998
Publication title -
crystal research and technology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.377
H-Index - 64
eISSN - 1521-4079
pISSN - 0232-1300
DOI - 10.1002/(sici)1521-4079(1998)33:4<569::aid-crat569>3.0.co;2-6
Subject(s) - diffraction , beam (structure) , optics , physics , bragg's law , degeneracy (biology) , bioinformatics , biology
The conventional modified 2‐beam treatment of dynamical X‐ray diffraction near a 3‐beam point is generalized by systematically including σ—π mixing in the normal modes. This generates an infinite set of first order modified 2‐beam solutions. The optimal among these removes most of the degeneracy that in the conventional formulation led to divergences in next order perturbation. Simultaneously, it also achieves a partial diagonalization that minimizes the remaining coupling between the new 2‐beam normal modes in the central region of the 2‐beam Bragg reflection. The σ—π contribution of the new modes varies with the angular distance of approach to the 3‐beam point. It yields an improved description of the effective 2‐beam dispersion surface, and it clarifies that even σ‐ or π‐polarized incident beams excite both normal modes within the crystal. The theory is compared with various earlier first order results, for both centric and non‐centrosymmetric structures, and is applied to new possibilities of invariant phase determination using multiple beam interactions.