Photoelectron Diffraction Determination of the Structure of Ultrathin Vanadium Films on Cu(001)

X-ray photoelectron diffraction (XPD) and low-energy electron diffraction (LEED) have been used to study the structural properties of V thin films on Cu(001). For room-temperature growth, submonolayer coverages result in (1x1) LEED patterns that evolve to exhibit very diffuse (2x1) structure at approximately 1 monolayer coverage. We do not observe any V forward-focusing enhancements for V films that exhibit either the (1x1) or (2x1) structure, suggesting that these structures are limited to the first 1-2 vanadium layers. At coverages above 1 monolayer, the V films display complex LEED patterns characteristic of four bcc(110) domains. This structure persists to V coverages as high as 100 ML, and the LEED and XPD angular scans suggest that V in these films retain the bulk V lattice constant. These results have important ramifications for predictions of magnetic order in vanadium thin films that typically assume pseudomorphic growth

A High Energy X-Ray and Neutron Scattering Study of Iron Phosphate Glasses Containing Uranium

The atomic structure of iron phosphate glasses containing uranium has been studied by complementary neutron and x-ray scattering techniques. by combining x-ray and neutron structure factors, detailed information about different pair interactions has been obtained. Most of the basic structural features such as coordination numbers and O-O and P-O distances in uranium containing glasses are the same as those in the base glass of batch composition 40Fe2O3-60P2O5 (mol %). However, the Fe-O distances change slightly with the addition of uranium. The observed structural parameters support a structural model in which the waste elements occupy voids in the Fe-O-P network, hence, not altering the basic structure of the parent iron phosphate glass

The Evolution of Ga and as Core Levels in the Formation of Fe/GaAs (001): A High Resolution Soft X-Ray Photoelectron Spectroscopic Study

A high resolution soft x-ray photoelectron spectroscopic study of Ga and as 3d core levels has been conducted for Fe/GaAs (001) as a function of Fe thickness. This work has provided unambiguous evidence of substrate disrupting chemical reactions induced by the Fe overlayer—a quantitative analysis of the acquired spectra indicates significantly differing behavior of Ga and as during Fe growth, and our observations have been compared with existing theoretical models. Our results demonstrate that the outdiffusing Ga and as remain largely confined to the interface region, forming a thin intermixed layer. Whereas at low coverages Fe has little influence on the underlying GaAs substrate, the onset of substrate disruption when the Fe thickness reaches 3.5 Å results in major changes in the energy distribution curves (EDCs) of both as and Ga 3d cores. Our quantitative analysis suggests the presence of two additional as environments of metallic character: one bound to the interfacial region and another which, as confirmed by in situ oxidation experiments, surface segregates and persists over a wide range of overlayer thickness. Analysis of the corresponding Ga 3d EDCs found not two, but three additional environments—also metallic in nature. Two of the three are interface resident whereas the third undergoes outdiffusion at low Fe coverages. Based on the variations of the integrated intensities of each component, we present a schematic of the proposed chemical makeup of the Fe/GaAs (001) system

Spin Spectrometer at the ALS and APS

A spin-resolving photoelectron spectrometer, the"Spin Spectrometer," has been designed and built. It has been utilized at both the Advanced Light Source in Berkeley, CA, and the Advanced Photon Source in Argonne, IL. Technical details and an example of experimental results are presented here

Direct Comparison of the X-Ray Emission and Absorption of Cerium Oxide

X-ray emission spectroscopy and x-ray absorption spectroscopy have been used to investigate the photon emission and absorption associated with the Ce 3d5/2 and Ce 3d3/2 core levels in Ce oxide. A comparison of the two processes and their spectra will be made

Ultrathin Vanadium Films on Cu(001): Structure and Magnetism

X-ray photoelectron diffraction (XPD), low energy electron diffraction (LEED), and magnetic linear dichroism in angular distributions of photoelectrons (MLDAD) have been used to study the structural and magnetic properties of V thin films on Cu(001). For room-temperature growth, XPD and LEED data indicate that below 1 monolayer (ML), equivalent V films exhibit a (2x1) structure that evolves to four domains of bcc (110) at coverages above 1 ML. This multi-domain structure produces a LEED pattern often referred to as \u27(3x1)\u27. This structure persists to V coverages as high as 100 ML, and the LEED and XPD angular scans indicate that V in these films retains the bulk V lattice constant. MLDAD results for 1-5 ML V films at room temperature and at 150 K provide no evidence for in-plane magnetic ordering. Annealing to 450°C results in V clustering at low coverages accompanied by Cu diffusion. For thick V films, the primary affect of annealing is interdiffusion of Cu and V

The Evolution of Ga and as Core Levels in the Formation of Fe/GaAs (001): A High Resolution Soft X-Ray Photoelectron Spectroscopic Study

A high resolution soft x-ray photoelectron spectroscopic study of Ga and as 3d core levels has been conducted for Fe/GaAs (001) as a function of Fe thickness. This work has provided unambiguous evidence of substrate disrupting chemical reactions induced by the Fe overlayer—a quantitative analysis of the acquired spectra indicates significantly differing behavior of Ga and as during Fe growth, and our observations have been compared with existing theoretical models. Our results demonstrate that the outdiffusing Ga and as remain largely confined to the interface region, forming a thin intermixed layer. Whereas at low coverages Fe has little influence on the underlying GaAs substrate, the onset of substrate disruption when the Fe thickness reaches 3.5 Å results in major changes in the energy distribution curves (EDCs) of both as and Ga 3d cores. Our quantitative analysis suggests the presence of two additional as environments of metallic character: one bound to the interfacial region and another which, as confirmed by in situ oxidation experiments, surface segregates and persists over a wide range of overlayer thickness. Analysis of the corresponding Ga 3d EDCs found not two, but three additional environments—also metallic in nature. Two of the three are interface resident whereas the third undergoes outdiffusion at low Fe coverages. Based on the variations of the integrated intensities of each component, we present a schematic of the proposed chemical makeup of the Fe/GaAs (001) system

X-ray Photoelectron and Mössbauer Spectroscopic Studies of Iron Phosphate Glasses Containing U, Cs and Bi

The atomic structure of iron phosphate glasses and those containing common high level nuclear waste components such as UO2, Cs2O and Bi2O3 has been investigated using X-ray photoelectron spectroscopy (XPS) and Mössbauer spectroscopy. Oxygen 1s spectra indicate that only 16-26% of the oxygen ions are bridging oxygens (BOs). The addition of waste components such as UO2, Cs2O and Bi2O3 does not change the BO/(BO + NBO) ratio appreciably. Some structural similarities in the short range order was found between the glass and crystalline Fe3(P2O7)2 which crystallizes from the glass upon heat treatment. The Mössbauer hyperfine parameters, isomer shift and quadrupole splitting, showed that the environment around iron ions was independent of the composition. In general, the addition of waste elements does not alter the basic features of the parent glass. The measured isomer shifts and quadrupole splittings indicate that the Fe3+ and Fe2+ ions in these glasses are in octahedral or distorted octahedral coordination. The fraction of Fe2+ in the glass increases with the melting temperature. However, most physical properties of these glasses do not depend appreciably on the valence state of iron ions

Local Environment of Iron and Uranium Ions in Vitrified Iron Phosphate Glasses Studied by Fe K and ULIII-edge X-Ray Absorption Fine Structure Spectroscopy

The local structure of iron and uranium ions in a series of iron phosphate glasses with the general composition (40 - x)Fe2O3-xUO2-60P2O5 and (1-x-y)(40Fe2O3-60P2O5)-xUO2-y(Na2O or CaO) was investigated using Fe K-edge and U LIII-edge x-ray absorption fine structure spectroscopy. Replacing Fe2O3 by UO2 in the glass caused more distortion in the coordination environment of Fe(III) ions. Extended x-ray absorption fine structure fits revealed that the Fe-P bonds observed in the base glass also existed in all the waste-loaded glasses. X-ray absorption near-edge structure showed that the uranium ions were predominantly present as U(IV) in the glasses. Uranium ions were coordinated to approximately 8 ± 1 oxygen atoms and 2.5 ± 0.6 phosphorus atoms at an average distance of 2.47 ± 0.02 and 3.8 ± 0.02 Å, respectively. There were no Fe-U or U-Fe neighbors observed, indicating that uranium ions occupied voids in the glass away from the PO4 units. These conclusions were supported by Mössbauer, x-ray photoelectron, and Raman spectroscopic data

A Cerium M-Edge X-ray Absorption Study of the CeM₂ Compounds, Where M Is Mg, Al, Fe, Co, Ni, Ru and Rh

The cerium valence in the CeM2 compounds, where M is magnesium, aluminum, iron, cobalt, nickel, ruthenium and rhodium, has been measured at room temperature by X-ray absorption at the cerium M-edge. The observed valencies, which agree with previous more limited measurements made at the cerium LIII-edge, extend the study to additional compounds and lead to the conclusion that the relationship between the cerium volume and its valence in the CeM2 compounds is not straightforward. The results show a clear linear increase in the cerium valence with the atomic number of the metal M and indicate that the increase in the cerium valence from 3.05 in CeMg2 to 3.24 in CeRh2 results from an increase in the hybridization between the cerium 4f orbitals and the highest occupied M orbitals