DFT and Plasmon-Coupling Models for Optical and Electronic Scattering Properties

We present calculations and applications of optical energy loss data for use in studies of inelastic electron scattering in condensed matter systems. A new model of plasmon coupling and excitation broadening is implemented along with high-precision density functional theory to evaluate fundamental material properties critical to many areas of spectroscopic analysis. Recent developments in x-ray and electron spectroscopies have demonstrated critical dependence on low-energy electron scattering and optical loss properties, and significant discrepancies between theoretical and experimental scattering values [1]. Resolution of these discrepancies is required to validate experimental studies of material structures, and is particularly relevant to the characterization of small molecules and organometallic systems for which electron scattering data is often sparse or highly uncertain [2]. We have devised a new theoretical approach linking the optical dielectric function and energy loss spectrum of a material with its electron scattering properties and characteristic plasmon excitations. For the first time we present a model inclusive of plasmon coupling, allowing us to move beyond the longstanding statistical approximation and explicitly demonstrate the effects of band structure on the detailed behavior of bulk electron excitations in a solid or small molecule. This is a novel generalization of the optical response of the material, which we obtain using density functional theory [3]. We find that our developments improve agreement with experimental electron scattering results in the low-energy region (<~100 eV) where plasmon excitations are dominant; a region that is particularly crucial for structural investigations using x-ray absorption fine structure and electron diffraction. This work is further relevant to several commissions of the IUCr including the commissions on XAFS, International Tables, and Electron Crystallography.

An XAFS Investigation of static and thermal disorder above the Nickel K-edge

We present recent experimental X-ray Absorption Fine Structure (XAFS) data of the Nickel K-edge, measured at temperatures of 15, 70 and 140 K. This study has taken elements of the X-ray Extended Range Technique (XERT) and for the first time, applied them to a cryostat cold cell system. These measurements permit critical tests of XAFS theory, with emphasis on quantification of the Debye-Waller factor and static vs. thermal disorder. X-ray Absorption Fine Structure contains vital information about the surrounding system of an absorbing atom including crystal structure, bond distances and coordination number. It is crucial that we understand all processes that may affect the measured XAFS spectra. The aim of this study is to investigate thermal effects and quantify thermal and static disorder [1]. The XERT is an experimental technique developed by our group, capable of measuring X-ray mass attenuation coefficients on an absolute scale with accuracies down to 0.02% [2]. This study has taken crucial elements from the XERT and applied them to complex experimental systems. This includes, but is not limited to high accuracy energy calibration [3], quantification and correction of beam harmonics and fluorescence. Our robust technique allows us to take the high accuracy data required to determine fundamental structural and crystallographic properties. These developments give great insight into our understanding of more complex systems such as organometallic molecules and biological systems.

Redox modulation of Low-Volume (100 µL) Solutions for XAFS measurements

A key to the understanding of transition metal catalysis is a detailed knowledge of the changes in coordination environment that accompany a change in redox state. The capacity of a given metal complex to support the high rates of electron transfer needed for effective catalysis is strongly dependent on the magnitude of structural reorganization coupled to the redox step. The ability of the ligand to control the dynamics of electron transfer is beautifully illustrated by copper redox proteins such as plastocyanin.[1] The polypeptide-imposed constraints on the environment at the coordination site of the metal minimize the structural change attendant on interconversion between the CuI and CuII redox states of the metal, facilitating fast electron transfer. Further, unravelling the molecular details of enzyme catalysis often hinges on knowledge of the structural changes attendant on oxidation or reduction. XAFS can provide the key structural information for reactive or unstable redox states of biological and abiological molecules. Our research has mostly centred on the use of a combination of spectroscopic and computational techniques to reveal the chemistry associated with dihydrogen activation in diiron compounds related to [FeFe]-hydrogenases [2], where a combination of XAFS, IR spectroscopy and theory can provide reliable structural information. Sampling of such species can provide a comparable challenge to spectral analysis - an issue made more difficult in cases where the quantity of sample is limited. We have developed low-volume electrosynthesis cells suitable for the study of electrogenerated species where the total volume of solution required for XAS data collection is of order 100 µL [3]. The design and operation of cells designed to allow freeze quenching and tow-temperature spectral collection or RT on-line measurement will be described.

Volume I: X-ray Absorption Spectroscopy, International Tables

Research in core physics and atomic and condensed matter science are increasingly relevant for diverse fields and are finding application in chemistry, engineering and biological sciences, linking to experimental research at synchrotrons, reactors and specialized facilities. A plethora of different approaches are popular in the literature and the Volume will hope to capture their greatest achievements and value by representation of all the leading groups from Europe, America, Asia, Australia plus elsewhere! Specifically, common elements and novel elements of XAFS, XANES, EXAFS, RIXS and diverse and related techniques will be represented, together with historical perspectives and latest developments. Over recent synchrotron experiments and publications methods have developed for measuring the absorption coefficient far from the edge and in the XAFS (X-ray absorption fine structure) region in neutral atoms, simple compounds and organometallics reaching accuracies of below 0.02%. This is 50-500 times more accurate than earlier methods, and 50-250 times more accurate than claimed uncertainties in theoretical computations for these systems. The data and methodology are useful for a wide range of applications, including major synchrotron and laboratory techniques relating to fine structure, near- edge analysis and standard crystallography. A comment on some key features of the new Volume in its infancy are presented, and contributions, support and suggestions will be warmly welcomed by all Editors.