Lars R. Furenlid

Use of X-ray absorption spectroscopy and esterification to investigate Cr(III) and Ni(II) ligands in alfalfa biomass

Previously performed studies have shown that alfalfa shoot biomass can bind an appreciable amount of nickel(II) and chromium(III) ions from aqueous solution. Direct and indirect approaches were applied to study the possible mechanis ms involved in metal binding by the alfalfa biomass. The direct approach involves investigations of the metal-bound alfal fa shoot biomass by X-ray absorption spectroscopic analysis (XANES and EXAFS). Results from these studies suggest that ni ckel(II) and chromium(III) binding mostly occurs through coordination with oxygen ligands. Indirect approaches consist of chemical modification of carboxylate groups that have been shown to play an important role in metal binding to the alfal fa biomass. An appreciable decrease in metal binding resulted after acidic methanol esterification of the biomass, indica ting that carboxyl groups are entailed in the metal binding by the alfalfa biomass. In addition, base hydrolysis of the a lfalfa biomass increased the binding of these metals, which further indicates that carboxyl groups play an important role in the binding of these metal ions from solution. Therefore, by combining two different techniques, our results indicate that carboxylate groups are the major ligands responsible for the binding of nickel(II) and chromium(III) by alfalfa bio mass.

XAS studies of Ni(I), Ni(II), and Ni(III) complexes

Abstract XAS techniques for studying structural and electronic changes taking place after oxidation and reduction reactions of nickel complexes are described and applied to selected models for Factor 430, a nickel containing cofactor catalyzing an important step in the conversion of carbon dioxide to methane by methanogenic bacteria.

A cell for extended x-ray absorption fine structure studies of oxygen sensitive products of redox reactions

We describe a cell suitable for extended x‐ray absorption fine structure (EXAFS) studies of oxygen and/or water sensitive products of redox reactions. The cell utilizes aluminized Mylar windows that are transparent to x rays, provide low gas permeability, and allow vacuum to be maintained in the cell. The windows are attached to the glassware with an epoxy that resists attack by common organic solvents. Additional side arms allow multiple spectroscopic probes of the same sample under anaerobic and anhydrous conditions.

New results with silicon pad detectors and low-noise electronics for absorption spectrometry☆☆☆

Abstract High-resistivity silicon diode detectors integrable in array structures have been tested for high-resolution X-ray spectroscopy at various temperatures. Using an inexpensive commercially available front-end transistor we have obtained resolutions of 19 electrons rms, or 165 eV FWHM, at −35°C and 39 electrons rms, or 320 eV FWHM, at 26°C, with 3 and 1 μs shaping times. The obtained resolution values are adequate for synchrotron light based EXAFS experiments. These reliable detectors can be suitably employed for high-performance multichannel systems in which high-rate capability is achieved by using an array of detectors operated in parallel. A 16-element detector prototype has been recently assembled, installed and successfully operated for a real EXAFS experiment at NSLS.

Multichannel analyzer/detector system for high-speed high-resolution powder diffraction

The advantage of using high-resolution powder diffraction data to resolve ambiguities in powder diffraction analyses is widely understood. The problem with using such an instrument for routine analyses is the length of time needed to acquire a data set. An obvious solution is to introduce some parallelism in the detector/analyzer system. The system described here uses monolithic elastic design principles to construct a simple 16-element crystal analyzer, and it uses silicon multi-detector technology developed at Brookhaven to provide a compact 16-channel pulse-counting detector system. Together these components can decrease the data collection time by at least a factor of 10. Simple replication of components could further increase the efficiency of collection.

High‐resolution multielement solid‐state detectors (invited)

Recent advances in multielement solid‐state detector systems for high rate, high resolution x‐ray spectroscopy at noncryogenic temperatures will be described in this paper. A 16‐channel silicon detector system, designed and built at BNL, has been recently operated in the NSLS machine beam ♯X19A, showing an average energy resolution of less than 250 eV FWHM, which is adequate to discriminate the fluorescence trace element against the background of elastically scattered photons in a typical EXAFS application. A larger, 128 channel system, will soon permit a higher overall count rate: ≳106 counts per second. It is shown that, in order to achieve high resolution with a solid‐state detector, special care must be spent in the detector‐preamplifier assembly. A low noise detectorpreamplifier may be obtained integrating the front‐end devices (an FET and/or a feedback capacitor) with the detector itself.

A VARIATION OF THE CROSS CORRELATION TECHNIQUE

A simplified, but versatile, cross-correlation technique is described. Suitable applications are determinations of radial velocities and searches for binary-star secondaries as well as interpretations of complicated spectra. The small number of operations used in defining the cross-correlation function allows the technique to be implemented in an on-line mode.

Photochemical carbon dioxide reduction with metal complexes: Differences between cobalt and nickel macrocycles

Problems related to increases of green house gases in the atmosphere and the depletion of fossil fuels have made the conversion of CO{sub 2} into useful chemicals and fuels an important area of research. However, CO{sub 2} reduction poses many scientific challenges. Despite intense interest in photochemical and electrochemical CO{sub 2} reduction, the kinetics and mechanism of the reduction remain unclear in many systems. This research focuses on mechanistic and kinetic studies of photochemical and electrochemical CO{sub 2} reduction that involves metal complexes as catalysts. This work makes use of UV-vis, NMR, and FTIR spectroscopy, flash photolysis, pulse radiolysis, X-ray diffraction, XANES (X-ray absorption near-edge spectroscopy) and EXAFS (extended X-ray absorption fine structure). Here the authors summarize their research on photochemical carbon dioxide reduction with metal macrocycles.

Damage observations on synchrotron beam line mirrors

Catastrophic damage has been observed in ZERODUR mirrors used as first mirrors in two beam lines at the National Synchrotron Light Source. ZERODUR was selected as a substrate for these uncooled, grazing incidence mirrors because of its superior thermal expansion properties near room temperature. Despite the high reflectivity of the coatings used on these mirrors, a significant flux of high energy photons penetrates below the coating and is absorbed mainly in a thin layer at the top of the substrate. Over a long time period the absorbed flux causes the glass material to compact, leading to a build-up of surface stress, gross figure changes, and the growth of fractures. The total dose of absorbed radiation for these mirrors is estimated to be in the range of 106 MRads, i.e. 1012 Rads, which is orders of magnitude greater than the dose used in conventional radiation damage studies for space-borne optics and deep UV lithography systems.

X-ray-induced damage observations on ZERODUR mirrors

Catastrophic damage has been observed in some ZERODUR mirrors used as first mirrors in two beam lines at the National Synchrotron Light Source. Despite the high reflectivity of the coatings used on these mirrors, a significant flux of high energy photons penetrates below the coating and is absorbed in the substrate. Although model calculations indicate that the local temperature does not increase significantly, we suspect that over long time periods the absorbed flux produces structural changes in the material, leading to a build-up of surface stress, gross figure changes, and growth of fractures. These changes are probably related to the nature of the two-phase glass- ceramic composition of the ZERODUR material. Metal mirrors and single-phase materials do not exhibit such catastrophic damage under similar exposure conditions.