Effect of Dynamical Diffraction in X-Ray Fluorescence Scattering

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Abstract
With a conventional double-crystal spectrometer, we have obtained direct evidence for the formation of the two types of standing-wave fields formed within the crystal during the diffraction process. As a germanium crystal was rotated through the Bragg-reflection region the fluorescence scattering as well as the diffracted beam was measured. The reflected beam gives the expected Darwin-Prins curve, while the fluorescence curve (a dip at the Bragg angle), is asymmetric with a long tail on the low-angle side. The fluorescence, in this case, is used as a measure of the electric-field intensity at the atomic electrons, and the asymmetry implies that there is less x-ray intensity in the diffracting planes of atoms at low glancing angles than there is at high angles. This is consistent with the dynamical theory prediction of the formation of a nodal plane of the x-ray wave field at the atoms at low glancing angles and an antinode at high angles. We show that the antinode forming at the high-angle side can produce a marked enhancement of the fluorescences over that produced when no diffraction is taking place. Very good agreement between experimental fluorescence intensities and that predicted by dynamical theory was obtained.