Extended X-ray absorption fine structure

About: Extended X-ray absorption fine structure is a research topic. Over the lifetime, 10452 publications have been published within this topic receiving 276744 citations.

Reduced derivatives of the Mn cluster in the oxygen-evolving complex of photosystem II: An EXAFS study

Abstract: X-ray absorption spectroscopy (XAS) has been used to characterize the local structural environment of the Mn in the resting (S{sub 1}) state and two different reduced derivatives of the photosynthetic oxygen-evolving complex (OEC). Short-term incubation with NH{sub 2}OH gives a state with minimal structural rearrangement relative to the S{sub 1} state, consistent with the small shift in X-ray absorption edge energy for the NH{sub 2}OH reduced sample. In contrast, hydroquinone reduced samples show significant structural rearrangements, including the appearance of a new Mn-O shell at 2.17 A and a decrease in the amplitude of the 2.7 A Mn...Mn interaction. The effects of both NH{sub 2}OH and hydroquinone are reversed by illumination and dark adaptation, indicating that the reductant induced changes are not the result of sample decomposition. Long-term incubation with NH{sub 2}OH and short-term incubation with higher concentrations of NH{sub 2}OH both result in greater reduction, more extensive structural change, and loss of activity. There is a linear correlation between the activity of these highly reduced samples and the Mn content of the samples. However, the activity per Mn atom remains constant, demonstrating that all of the Mn detected by XAS is present in active OEC centers. This demonstrates thatmore » highly reduced centers can be produced not only by hydroquinone but also by NH{sub 2}OH. A refined mechanism for reduction of Mn in the OEC is proposed. 111 refs., 10 figs., 6 tabs.« less

Determination of Scattering and Absorption Coefficients for Plasma‐Sprayed Yttria‐Stabilized Zirconia Thermal Barrier Coatings at Elevated Temperatures

Abstract: The temperature dependence of the scattering and absorption coefficients for a set of freestanding plasma-sprayed 8 wt% yttria-stabilized zirconia (8YSZ) thermal barrier coatings (TBCs) was determined at temperatures up to 1360 C in a wavelength range from 1.2 micrometers up to the 8YSZ absorption edge. The scattering and absorption coefficients were determined by fitting the directional-hemispherical reflectance and transmittance values calculated by a four-flux Kubelka Munk method to the experimentally measured hemispherical-directional reflectance and transmittance values obtained for five 8YSZ thicknesses. The scattering coefficient exhibited a continuous decrease with increasing wavelength and showed no significant temperature dependence. The scattering is primarily attributed to the relatively temperature-insensitive refractive index mismatch between the 8YSZ and its internal voids. The absorption coefficient was very low (less than 1 per centimeter) at wavelengths between 2 micrometers and the absorption edge and showed a definite temperature dependence that consisted of a shift of the absorption edge to shorter wavelengths and an increase in the weak absorption below the absorption edge with increasing temperature. The shift in the absorption edge with temperature is attributed to strongly temperature-dependent multiphonon absorption. While TBC hemispherical transmittance beyond the absorption edge can be predicted by a simple exponential decrease with thickness, below the absorption edge, typical TBC thicknesses are well below the thickness range where a simple exponential decrease in hemispherical transmittance with TBC thickness is expected. [Correction added after online publication August 11, 2009: "edge to a shorter wavelengths" has been updated as edge to shorter wavelengths."]

Zinc adsorption on clays inferred from atomistic simulations and EXAFS spectroscopy.

Abstract: Clay minerals are efficient sinks for heavy metals in the geosphere. Knowing the uptake mechanism of these elements on clays can help to protect the natural environment from industrial pollution. In this study ab initio molecular dynamics (MD) calculations were applied to simulate the uptake of Zn on the edge surfaces of montmorillonite, a dioctahedral clay, and to explain the measured K-edge extended X-ray absorption fine structure (EXAFS) spectra of adsorbed Zn. These experiments were carried out using a high ionic strength Na background electrolyte that enables one to block cation exchange processes and to restrict the Zn uptake to the sorption complexation at the edge sites of clay. The analysis of the experimental data and simulation results suggest that structurally incorporated Zn preferentially substitutes for Al(III) in the trans-symmetric sites of the octahedral layer. At low loading, Zn is incorporated into the outermost trans-octahedra on (010) and (110) edges. At medium loading, Zn forms mono...