J. L. Glover

Measurement of the x-ray mass-attenuation coefficients of gold, derived quantities between 14 keV and 21 keV and determination of the bond lengths of gold

The x-ray mass-attenuation coefficients of gold are measured at 91 energies between 14 keV and 21 keV using synchrotron radiation. The measurements are accurate to between 0.08% and 0.1%. The photoelectric mass-absorption coefficients and the imaginary component of the form factors of gold are also determined. The results include the LI edge and are the most accurate and extensive gold dataset available in this energy range. An analysis of the LI edge XAFS showed excellent agreement between the measured and simulated XAFS and yielded highly accurate values of the bond lengths of gold. When our results are compared with earlier measurements and with predictions of major theoretical tabulations, significant discrepancies are noted. The comparison raises questions about the nature of discrepancies between experimental and theoretical values of mass-attenuation coefficients.

The analysis of x-ray absorption fine structure: beam-line independent interpretation

Can current experimental techniques and analytical procedures produce x-ray absorption fine structure (XAFS) which is independent of the beam line or synchrotron used? We investigate the consequence upon XAFS interpretation of typical systematic errors, including determination of the edge energy, detector response and synchrotron bandwidth. Using the highest accuracy data set of the mass-attenuation coefficient collected so far, we consider a series of systematic effects in the analyses of both the near-edge and extended energy regions of the spectrum. We investigate whether conclusions derived from an experiment using a given analytical procedure are consistent when performed on different synchrotron beam lines. We find that the effectiveness of common XAFS analysis is limited by experimental and data reduction techniques, particularly relating to determinations of photon energy. By correcting for all major systematic errors in XAFS data, one can determine bond lengths more robustly and with greater accuracy.

Measuring the linearity of X-ray detectors: consequences for absolute attenuation, scattering and absolute Bragg intensities

The linearity of response of X-ray detectors is tested. Examples of linearity tests demonstrate the remarkable range of linear response of flowing-gas ion chambers in the synchrotron environment. The diagnostic is also highly sensitive to the presence in the X-ray beam of harmonic X-rays diffracted by a higher-order reflection of the monochromator. The remarkable range of linearity of ion chambers has enabled the accurate measurement of the absolute X-ray attenuation of a number of elements. It should now be possible to measure the absolute intensity of Bragg reflections, provided such measurements are carried out with extended-face single crystals. The advantages of the extended-face crystal technique for Bragg intensity measurements are summarized and a number of approaches to absolute Bragg intensity measurement are discussed.

Theoretical XANES study of the activated Nickel (t-amylisocyanide) molecule

XANES is one of the most powerful techniques for investigating the active centres of non‐crystalline systems such as synthetic catalysts and enzymes. We have investigated XANES for an active species in the Ni‐catalyzed polymerization of isocyanides, the activated Ni (t‐amylisocyanide) complex, using two of the most popular theoretical approaches. This is a very large cluster for which it is extremely difficult to derive a converged solution using the Finite Difference Method. The cluster has been linked to important chemical developments for catalysts for isocyanide polymerization. Predicted XANES for the nano‐cluster are compared with experimental data, providing an important test for different theoretical approaches. Developments of a finite element method gave excellent agreement with the experimental data, while simpler models were relatively unsuccessful.