![]() However, isolation of the intensities arising from all individual RLPs is necessary if the corresponding structure amplitudes |F(hkl)|, required for all subsequent quantitative crystallographic analyses, are to be obtained. The total intensity of a Laue spot is the sum of the intensities arising from each RLP within that spot. Multiple Laue spots thus contain more than one X-ray energy. Each RLP within a multiple spot selects the appropriate X-ray energy according to Bragg's Law from the polychromatic X-ray spectrum. The individual orders overlap exactly on the detector and cannot be spatially separated. (2h2k2 l) and (3h3k3 l) or many more than two the spot is said to be multiple, containing multiple orders. with indices (hkl) or (3h3k3 l) the spot is said to be single, containing a single order. For a given X-ray beam and crystal resolution, the number of RLPs on each central ray and their specific indices that satisfy these constraints varies with crystal orientation. Each RLP within this volume selects the X-ray energy that satisfies Bragg's Law for that RLP. The mapping is thus quite different for monochromatic and Laue diffraction.Įach Laue spot associated with a central ray contains contributions from all RLPs that lie in the volume between three limiting spheres: the Ewald spheres corresponding to 1/ λ min and 1/ λ max, and a sphere centred on the origin representing the limiting resolution of the crystal, d* max. Laue diffraction maps central rays on to the X-ray detector. they have no common factor greater than 1 (hkl) denotes a first order RLP, (2h2k2 l) a second order RLP, and so on. In the array, h, k and l are co-prime, i.e. (nhnknl) … This line is known as a central ray. These RLPs all lie on a line in reciprocal space passing through the origin (000) and through a radial array of RLPs: (hkl), (2h2k2 l), …. By contrast, for a Laue diffraction pattern, several RLPs may be associated with each Laue spot (e.g. That is, monochromatic diffraction maps individual RLPs on to the X-ray detector that records the pattern. The intensity of that spot can be accurately measured and the desired structure amplitude directly extracted. For a monochromatic diffraction pattern, the relationship is relatively straightforward: each RLP is associated with a single diffraction spot in the overall pattern. However, the relationship between diffraction spots and RLPs differs between monochromatic and Laue diffraction. In monochromatic diffraction (now much more widely used), the X-rays have a single X-ray energy/wavelength λ and the crystal is rotated during the exposure.įor structure determination of the molecules in all but the very simplest crystals, the structure amplitudes |F(hkl)| associated with each reciprocal lattice point (RLP) with indices hkl must be accurately extracted from the intensities of the diffraction spots. In Laue diffraction, the X-rays are polychromatic with wavelengths ranging from λ min to λ max and the crystal is stationary throughout the exposure. During the earliest years of crystallography by Bragg, Dickinson and Dickinson's student Pauling, Laue diffraction was widely used for semi-quantitative structure determination of structures containing only a few atoms. This article is part of the theme issue ‘Fifty years of synchrotron science: achievements and opportunities’.ĭiffraction of X-rays occurs when a beam of X-rays falls on a crystal. However, they too are being applied to time-resolved crystallography to explore, for example, isomerization and rapid tertiary structural changes on the chemical, femtosecond timescale. The femtosecond X-ray pulses from such sources are completely destructive, generate only one diffraction pattern per tiny crystal and have unusual properties. Most recently, hard X-ray free electron laser sources have been used to generate narrow bandpass Laue diffraction patterns. Laue diffraction has been successfully applied at storage ring sources to time-resolved, pump–probe crystallographic studies, whose exposure time and time resolution were progressively reduced from minutes to seconds, milliseconds, nanoseconds and 100 ps. However, the advent of naturally polychromatic, intense, pulsed storage ring X-ray sources in the 1970s led to re-examination at Daresbury and elsewhere of its underlying principles. Laue diffraction has inherent complications largely absent in monochromatic diffraction, and consequently fell into disuse for quantitative structure determination. In Laue diffraction, a stationary crystal is illuminated by a polychromatic X-ray source. A personal, historical view is presented of Laue X-ray diffraction and its application to time-resolved studies of dynamic processes, largely in light-sensitive biological systems.
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