Showing papers by "David K. Shuh published in 1996"

Resonant X-Ray Fluorescence Spectroscopy of Correlated Systems: A Probe of Charge-Transfer Excitations

Abstract: X-ray fluorescence spectroscopy with monochromatic photon excitation is presented as a tool for studies of charge-transfer excitations in correlated systems, using CeO{sub 2} and UO{sub 3} as examples. Ce 4{ital f}{r_arrow}3{ital d} and U 5{ital f}{r_arrow}3{ital d} x-ray fluorescence, with excitation near the 3{ital d} thresholds, probes states as eigenvalues for the ground state Hamiltonian from the Anderson impurity model. Sweeping the excitation energy across 3{ital d} absorption edges enhances contributions of different electronic configurations to fluorescence so that observed resonances indicate the charge-transfer origin of the absorption satellites. {copyright} {ital 1996 The American Physical Society.}

Characterization of nanocrystalline diamond films by core‐level photoabsorption

Abstract: Core‐level photoabsorption has been used to determine the sp2 and sp3 bonding content of nanocrystalline diamond thin films grown using C60 or CH4 precursors. The C(1s) absorption spectra show clear bulk diamond excitonic and sp3 features with little evidence of sp2 bonding, while the Raman spectra measured from these same films are ambiguous and indeterminate. This result can be attributed to the local structure (near‐neighbor bonding) sensitivity of core‐level photoabsorption that is insensitive to domain size, unlike Raman spectroscopy.

Photo‐oxidation of electroluminescent polymers studied by core‐level photoabsorption spectroscopy

Abstract: The C 1s and O 1s core‐level photoabsorption spectra of poly[2‐methoxy,5‐(2′‐ethyl‐ hexoxy)‐1,4‐phenylene vinylene] (MEH‐PPV) before and after exposure O2 and broadband visible light were recorded to determine the degradation pathway for this polymer. The change in the O 1s spectra as a function of exposure demonstrates that the O adds to the polymer chain to form a carbonyl group. Exposure to only O2 or only light causes no change in the C 1s or O 1s spectra. In the C 1s spectra, the change in the dependence on the photon angle of incidence after exposure demonstrates that O attacks the polymer at the double bond in the vinyl group thereby altering the extended conjugation of the polymer.

EXAFS Determinations of Uranium Structures: The Uranyl Ion Complexed with Tartaric, Citric, and Malic Acids

Abstract: U LIII-edge EXAFS spectra were measured for 1:1 mixtures of the uranyl ion with tartaric, malic, and citric acids. The room-temperature EXAFS spectra of these mixtures demonstrate the characteristic uranium−oxygen interactions of the uranyl ion UO22+ and also show a U−U interaction at 3.92 A, indicating the presence of a uranyl dimer. On the basis of models proposed by Martell et al. from earlier potentiometric titration experiments, it is concluded that the dimer is the ligand-bridged species (UO2)2(L)2, in which bridging occurs through the α-hydroxyl groups of the polycarboxylate ligands.

Near‐edge x‐ray absorption fine structure study of bonding modifications in BN thin films by ion implantation

Abstract: Near‐edge x‐ray absorption fine structure (NEXAFS) has been used to study the defect content and the bonding modifications induced in BN thin films by ion implantation. The initial films were hexagonal‐like BN grown on Si(100) by pulsed laser deposition. Subsequent ion implantation with N2+ at 180 keV induces the formation of a significant proportion of sp3 bonding (cubic‐like), and the formation of nitrogen void defects in the remaining sp2 BN. These modifications in the bonding of a film lacking long range order can only be distinguished with a local order technique like NEXAFS.

Characterization of Hydrous Uranyl Silicate by EXAFS

Abstract: Extended X-ray absorption fine structure (EXAFS) analysis was performed on uranyl orthosilicate, (U02)2Si04 · 2H20, and uranium(VI) sorbed onto silicic acid and silica gel. Uranyl orthosilicate was investigated as a reference for EXAFS studies of similar but non-crystalline uranium, oxygen, and silicon containing samples. Fitting the EXAFS spectrum yields the following distances for the first four coordination shells of uranium: U— Oax = 1.79 Á, U-O e i l = 2.38 λ , U S i = 3.16 Â, and U U = 3.88 A. These values agree well with results from single-crystal X-ray diffraction (XRD) measurements. Structural parameters of light elements such as oxygen and silicon at distances greater than 3.5 A could not be detected without a priori knowledge of their presence. The EXAFS spectra of uranyl species sorbed at pH 4 onto silicic acid and silica gel are identical indicating similar uranyl coordination. The main characteristic of the surface species are two well-separated oxygen coordination shells in the equatorial uranyl plane at 2.27 and 2.50 A. The results of the EXAFS analysis favor the interpretation of the uranyl surface species as an inner-sphere, mononuclear, bidentate complex.

A multichannel monolithic Ge detector system for fluorescence x‐ray absorption spectroscopy

Abstract: Construction and performance of a monolithic quad-pixel Ge detector for fluorescence x-ray absorption spectroscopy (XAS) at synchrotron radiation sources are described. The detector semiconductor element has an active surface area of 4.0 cm{sup 2} which is electrically separated into four 1.0 cm{sup 2} pixels, with little interfacial dead volume. Spatial response of the array shows that cross-talk between adjacent pixels is < 10% for 5.9 keV photons that fall within 0.5 mm of the pixel boundaries. The detector electronics system uses pre-amplifiers built at LBNL with commercial Tennelec Model TC 244 amplifiers. Using an {sup 55}Fe test source (MnK{sub {alpha}}, 5.9 keV), energy resolution of better than 200 eV is achieved with a 4 {mu}sec peaking time. At 0.5 {mu}sec peaking time, pulse pileup results in a 75% throughput efficiency for an incoming count rate of 100 kHz. Initial XAS fluoresncece measurements at the beamline 4 wiggler end stations at SSRL show that the detector system has several advantages over commercial x-ray spectrometers for low-concentration counting.

Role of surface stoichiometry in the Cl2/GaAs(001) reaction

Abstract: The room‐temperature reaction of Cl2 with GaAs(001)‐4×6, ‐c(2×8), and ‐c(4×4) surfaces is studied with synchrotron soft x‐ray photoelectron spectroscopy. The chemical composition of the reacted surfaces is found to depend on the stoichiometry of the starting surface. In all cases, the reaction occurs stepwise, with Ga and As monochlorides formed prior to the dichlorides. The Ga‐rich surface is initially more reactive than either of the As‐rich surfaces and it forms more GaCl than the As‐rich surfaces, which instead form more AsCl. The sticking coefficient for chlorine on GaAs(001) decays exponentially with coverage. A contribution from Cl atoms comprising the surface dichlorides is identified in the Cl 2p core‐level spectra.

The temperature dependence of the Cl2/GaAs(110) surface product distribution

Abstract: The reaction of Cl2 with GaAs(110) is studied with soft x‐ray photoelectron spectroscopy (SXPS). The temperature dependence of the surface product distribution, in the range of 300–650 K, is derived from SXPS core‐level and valence‐band spectra and compared to known gas‐phase product distributions. It is found that both Ga and As chlorides are formed at room temperature. Following reaction at temperatures above ∼400 K, no Cl remains on the surface. Instead, for temperatures up to ∼600 K an As overlayer is formed, whereas reaction at ∼650 K leads to the stoichiometric removal of Ga and As. These findings indicate that there is a direct correlation between the surface and gas‐phase product distributions.

Characterization of the Interaction of Uranyl Ions with Humic Acids by X-Ray Absorption Spectroscopy

Abstract: Humic substances are present throughout the environment in soil and natural water. They are organic macromolecules with a variable structural formula, molecular weight, and a wide variety of functional groups depending on their origin. In natural waters, humic substances represent the main component of the “dissolved organic carbon” (DOC). The DOC may vary considerably from 1 mg/L at sea water surfaces to 50 mg/L at the surface in dark water swamps.1 There is strong evidence that all actinides form complexes with humic substances in natural waters.2 Therefore, humic substances can play an important role in the environmental migration of radionuclides by enhancing their transport. Retardation through humic substance interaction may be also possible due to formation of precipitating agglomerates. For remediation and restoration of contaminated environmental sites and risk assessment of future nuclear waste repositories, it is important to improve the predictive capabilities for radionuclide migration through a better understanding of the interaction of radionuclides with humic substances.

EXAFS spectroscopic studies of uranium(VI) oxide precipitates

Abstract: We have investigated the structures of U(VI) oxides precipitated from room temperature aqueous solutions at low ionic strength as a function of pH. Using the uranium L{sub III} - edge extended x-ray absorption fine structure (EXAFS) as a probe of the local structure around the uranium, a trend is observed whereby the axial oxygen bond lengths from the uranyl groups increase from 1.80 {Angstrom} at pH=7 to 1.86 {Angstrom} at pH=11. A concomitant decrease in the equatorial oxygen and nearest-neighbor uranium bond lengths also occurs with increasing pH. Expansion of the linear O=U=O group is seen directly at the L{sub III} absorption edge where multiple scattering resonances systematically shift in energy. EXAFS curve-fitting analysis on these precipitates and a sample of synthetic schoepite indicate that the structure of the species formed at pH=7 is similar to the structure of schoepite. At pH=11, the precipitate structure is similar to that of a uranate.

Application of Synchrotron Radiation Techniques to Chemical Issues in the Environmental Sciences

Abstract: The problem of hazardous waste contamination in the environment has become an issue of central importance in society. As the number of remediation efforts taking place worldwide grows and the public awareness of these problems increases, it is clear that the problems faced are enormous in terms of effort and cost. With existing chemical technologies being applied to well over one thousand contaminated sites within the U.S. alone, cost estimates for the cleanup efforts range well into the tens of billions of dollars. Furthermore, because many of the problems are still being assessed and identified, there is no accurate time estimate for the completion of these projects, with currently proposed restoration programs ranging well into the mid-21st century.

Near-edge x-ray absorption fine structure examination of chemical bonding in sputter deposited boron and boron-nitride films

Abstract: Near-edge x-ray absorption fine structure (NEXAFS) is used to examine the chemical bonding in boron and boron-nitride films sputter deposited from a fully-dense, pure boron target. Reactive sputtering is used to prepare the boron-nitride and multilayered films. Although the process of sputter deposition often produces films that lack long range order, NEXAFS reveals the distinguishing features of sp{sup 2} and sp{sup 3} hybridization that are associated with different crystalline structures. The sensitivity of NEXAFS to local order further provides details in bonding modifications that exist in these films.

Surface Physics with Synchrotron Radiation

Abstract: This article illustrates the use of synchrotron radiation for illuminating the electronic structure of surfaces. After a brief introduction into the most common spectroscopies, their capabilities are exposed by a set of examples from the authors’ area of expertise. It comprises the surface and interface chemistry of semiconductors and magnetic materials, but leaves out a large body of work on surface chemistry of heterogeneous catalysis.