Premium
CHOOCH : a program for deriving anomalous‐scattering factors from X‐ray fluorescence spectra
Author(s) -
Evans Gwyndaf,
Pettifer Robert F.
Publication year - 2001
Publication title -
journal of applied crystallography
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.429
H-Index - 162
ISSN - 1600-5767
DOI - 10.1107/s0021889800014655
Subject(s) - anomalous scattering , scattering , wavelength , spectral line , absorption edge , absorption (acoustics) , physics , x ray fluorescence , x ray , computational physics , fluorescence , atomic physics , crystallography , optics , chemistry , condensed matter physics , quantum mechanics , band gap
A Fortran program CHOOCH , which derives experimental values of the anomalous‐scattering factors f ′′ and f ′ from X‐ray fluorescence spectra, is described. The program assumes knowledge of theoretical values for the imaginary term, f ′′, of the anomalous‐scattering factor away from the absorption edge to scale the experimental fluorescence spectrum and thus derive values of f ′′ near the absorption edge, where tabular data are inappropriate. The Kramers–Kronig relation is used to calculate the real part, f ′, of the anomalous‐scattering factor. The program aids the decision‐making process in macromolecular crystallographic experiments where optimal wavelength selection is required. Magnitudes of f ′ and f ′′ at selected wavelengths can later be used as starting values for heavy‐atom refinement with crystallographic phasing programs.