A foil-mask spectrometer for Laue diffraction pattern imaging: Simultaneous position, intensity and energy

An X-ray spectrometer for simultaneous position, intensity and energy determinations suitable for Laue diffraction applications is described. The foil-mask spectrometer consists of a series of metal foils of varying composition and thickness which are used to modulate the energy distribution of an incident X-ray source. Three modes of operation are described: a high-resolution spectrometer for measurement of nearly monochromatic X-rays, an intensity discriminator for partitioning the intensity from a small number of spatially overlapped monochromatic X-ray sources, and a low-resolution spectrometer for polychromatic X-rays with broad spectral features. The first mode of operation is designed to allow the energy of monochromatic Laue reflections to be measured with a resolution suitable for determination of unit-cell parameters. The second mode of operation is designed to allow the intensity of each component in a spatial region containing overlapping orders or spatially overlapped reflections to be discriminated for use in refinements or space-group assignment. The third mode of operation is described for completeness. The theory behind each mode of operation is described. The energy resolution of the spectrometer improves with the square root of the intensity of the incident beam. It also increases linearly with the change in energy with respect to transmission efficiency of a particular foil. In theory, the resolution of the spectrometer can readily exceed 50 eV over a wide range of energies depending on the foils used and the incident X-ray photon flux. Determinations of the energies of Mo K alpha and Cu K alpha radiation using a first-generation ten-foil spectrometer gave values of 17.5 +/- 0.1 and 8.08 +/- 0.05 keV, respectively. Treatment of random error shows good correspondence with a Poisson model. The use of this spectrometer is demonstrated using a sample of tetraphenylphosphonium tetrachlorooxo-molybdenum(V). Comparison of predicted and observed energies shows good agreement over a wide range of energies. The ratio of predicted to measured energy for the first 50 measurements was 0.9918 +/- 0.0344. Up to three components of a position having harmonic overlap were separated. This work demonstrates the feasibility of using Laue diffraction to completely determine the crystal structure of a molecule without recourse to monochromatic methods.