Electronic structure of americium sesquioxide probed by resonant inelastic x-ray scattering

The Am 5d-5f resonant inelastic x-ray scattering (RIXS) data of americium sesquioxide were measured at incident photon energies throughout the Am O4,5 edges. The experiment was supported by calculations using several model approaches. While the experimental Am O4,5 x-ray absorption spectrum of Am2O3 is compared with the spectra calculated in the framework of atomic multiplet and crystal-field multiplet theories and Anderson impurity model (AIM) for the Am(III) system, the recorded Am 5d-5f RIXS data are essentially reproduced by the crystal-field multiplet calculations. A combination of the experimental scattering geometry and theoretical analysis of the character of the electronic states probed during the RIXS process confirms that the ground state of Am2O3 is singlet Γ1. An appearance of the low-intense charge-transfer satellite in the Am 5d-5f RIXS spectra at an energy loss of ∼5.5eV suggests weak Am 5f-O 2p hybridization which is in agreement with AIM estimations of the 5f occupancy from spectroscopic data in Am2O3 as being 6.05 electrons

EXAFS investigation of UF4

A comparative structural study of UF4 and UO2, using extended x-ray absorption fine structure (EXAFS) and x-ray absorption near edge structure (XANES) techniques was conducted as part of the studies of 5f covalency in uranium compounds. This study confirmed the quality of the samples and provided additional insight into geometrical issues related to ionicity and covalency. It was observed that the local structure of UF4 from EXAFS was consistent with the long range structure derived from diffraction data

Oxidant K edge x-ray emission spectroscopy of UF4 and UO2

The K-Edge (1s) x-ray emission spectroscopy of uranium tetrafluoride and uranium dioxide were compared to each other and to the results of a pair of earlier cluster calculations. Using a very simplified approach, it is possible to qualitatively reconstruct the main features of the x-ray emission spectra from the cluster calculation state energies and 2p percentages

Oxidant K edge x-ray emission spectroscopy of UF4 and UO2

The K-Edge (1s) x-ray emission spectroscopy of uranium tetrafluoride and uranium dioxide were compared to each other and to the results of a pair of earlier cluster calculations. Using a very simplified approach, it is possible to qualitatively reconstruct the main features of the x-ray emission spectra from the cluster calculation state energies and 2p percentages

INITIAL-STAGES OF ATOMIC-HYDROGEN CHEMISORPTION ON GAAS(110) - A HIGH-RESOLUTION PHOTOEMISSION-STUDY

We analyzed atomic hydrogen chemisorption on GaAs(110) surfaces through synchrotron radiation photoemission studies of core level and valence band emission in the 2200-8800 L exposure range. The variation of the surface shifted and hydrogen induced core components as a function of exposure indicates that hydrogen chemisorption involves the formation of both Ga-H and As-H bonds during the early stages of chemisorption, as observed at higher hydrogen exposures, although not all of the dangling bonds are saturated by hydrogen, and there is no evidence of preferential As desorption that takes place at higher hydrogen coverages. The simple electronegativity picture of H-GaAs bonding that would suggest a partially ionic character of the chemisorption bond with charge transfer from surface anions and cations to the hydrogen appears inconsistent with the observed variation in surface work function and electron affinity with hydrogen exposure

Chemical speciation of U, Fe, and Pu in melt glass from nuclear weapons testing

Nuclear weapons testing generates large volumes of glassy materials that influence the transport of dispersed actinides in the environment and may carry information on the composition of the detonated device. We determine the oxidation state of U and Fe (which is known to buffer the oxidation state of actinide elements and to affect the redox state of groundwater) in samples of melt glass collected from three U.S. nuclear weapons tests. For selected samples, we also determine the coordination geometry of U and Fe, and we report the oxidation state of Pu from one melt glass sample. We find significant variations among the melt glass samples and, in particular, find a clear deviation in one sample from the expected buffering effect of Fe(II)/Fe(III) on the oxidation state of uranium. In the first direct measurement of Pu oxidation state in a nuclear test melt glass, we obtain a result consistent with existing literature that proposes Pu is primarily present as Pu(IV) in post-detonation material. In addition, our measurements imply that highly mobile U(VI) may be produced in significant quantities when melt glass is quenched rapidly following a nuclear detonation, though these products may remain immobile in the vitrified matrices. The observed differences in chemical state among the three samples show that redox conditions can vary dramatically across different nuclear test conditions. The local soil composition, associated device materials, and the rate of quenching are all likely to affect the final redox state of the glass. The resulting variations in glass chemistry are significant for understanding and interpreting debris chemistry and the later environmental mobility of dispersed material

Surface degradation of uranium tetrafluoride

A detailed analysis of a single crystal of uranium tetrafluoride has been carried out. The techniques include x-ray absorption spectroscopy, as well as x-ray photoelectron spectroscopy and x-ray emission spectroscopy. Evidence will be presented for the presence of a uranyl species, possibly UO2F2, as a product of, or participant in the surface degradation

Isotopic determination of uranium in soil by laser induced breakdown spectroscopy

Laser-induced breakdown spectroscopy (LIBS) operated under ambient pressure has been evaluated for isotopic analysis of uranium in real-world samples such as soil, with U concentrations in the single digit percentage levels. The study addresses the requirements for spectral decomposition of 235U and 238U atomic emission peaks that are only partially resolved. Although non-linear least-square fitting algorithms are typically able to locate the optimal combination of fitting parameters that best describes the experimental spectrum even when all fitting parameters are treated as free independent variables, the analytical results of such an unconstrained free-parameter approach are ambiguous. In this work, five spectral decomposition algorithms were examined, with different known physical properties (e.g., isotopic splitting, hyperfine structure) of the spectral lines sequentially incorporated into the candidate algorithms as constraints. It was found that incorporation of such spectral-line constraints into the decomposition algorithm is essential for the best isotopic analysis. The isotopic abundance of 235U was determined from a simple two-component Lorentzian fit on the U II 424.437 nm spectral profile. For six replicate measurements, each with only fifteen laser shots, on a soil sample with U concentration at 1.1% w/w, the determined 235U isotopic abundance was (64.6 ± 4.8)%, and agreed well with the certified value of 64.4%. Another studied U line - U I 682.691 nm possesses hyperfine structure that is comparatively broad and at a significant fraction as the isotopic shift. Thus, 235U isotopic analysis with this U I line was performed with spectral decomposition involving individual hyperfine components. For the soil sample with 1.1% w/w U, the determined 235U isotopic abundance was (60.9 ± 2.0)%, which exhibited a relative bias about 6% from the certified value. The bias was attributed to the spectral resolution of our measurement system - the measured line width for this U I line was larger than its isotopic splitting. Although not the best emission line for isotopic analysis, this U I emission line is sensitive for element analysis with a detection limit of 500 ppm U in the soil matrix; the detection limit for the U II 424.437 nm line was 2000 ppm