A spectroscopic study of uranium(VI) interaction with magnetite

The uranium sorbed onto commercial magnetite has been characterized by using two different spectroscopic techniques such as X-ray photoelectron spectroscopy (XPS), and extended X-ray absorption fine structure (EXAFS). Magnetite samples have been put in contact with uranium(VI) solutions in conditions in which a high uranium uptake is expected. After several days, the magnetite surface has been analysed by XPS and EXAFS. The XPS results obtained are not conclusive regarding the uranium oxidation state in the magnetite surface. On the other hand, the results obtained with the EXAFS technique show that the uranium-magnetite sample spectrum has characteristics from both the UO₂ and schoepite spectra, e.g. a relatively high coordination number of equatorial oxygens and two axial oxygens, respectively. These results would indicate that the uranium sorbed onto magnetite would be a mixture of uranium(IV) and uranium(VI).     

X-ray photoelectron spectroscopy study of the chemisorption of water on uranium and thorium and oxygen on uranium

The initial stages of water and oxygen adsorption on clean uranium and thorium and oxygen adsorption on uranium has been studied by X-ray photoelectron spectroscopy. These measurements are made in the vicinity of 120 K; in some cases the effect of temperature was followed up to 500 K. Three oxygen (1s) photoelectron peaks are observed for water adsorbed on uranium and thorium. All three peaks have binding energies different from the adsorption of oxygen. The uranium and thorium ($\text{4}\text{f}{}_{\mathrm{<mtext>7</mtext><mtext>2</mtext>}}$) lines also have different binding energies for water adsorption than for oxygen adsorption. Oxygen and an OH complex combine with uranium and thorium in a complex way, precluding the initial formation of simple oxides and confirming the importance of OH in the interaction of water with uranium and thorium.     

Study of the initial stage of yttrium oxidation using characteristic electron-energy-loss spectroscopy

The characteristic electron-energy-loss (EEL) spectra of the pure surface of metallic yttrium and of this surface in the initial stages of oxidation are recorded. The energy of the primary electron beam E _p is 200–1000 eV. The spectra exhibit high-and low-frequency peaks. During oxidation, the positions of the basic peaks in the EEL spectra are significantly shifted. The peaks corresponding to the bulk energy loss shift toward higher energies upon oxidation. The peak corresponding to the low-frequency surface oscillations also shifts, but toward lower energies, and its intensity monotonically decreases with increasing oxygen dose. The differences between the spectra recorded at different E _p are explained as resulting from an increase in the electron escape depth with E _p .     

Auger electron spectroscopic study of the surface oxidation of uranium-niobium alloy {U-6 wt.% Nb} in a UHV environment containing primarily H2, H2O and CO

Auger electron spectroscopy has been used to study the surface oxidation of a uranium-niobium alloy, U-6 wt.% Nb, in a UHV environment containing primarily H₂, H₂O and CO. For comparison, the reaction of the oxidative species with pure uranium and niobium metals has also been investigated. The observed changes of the shape and position of the Auger transition peaks for U, Nb and O following exposures this UHV environment up to 200 L, and under a continuous electron bombardment indicated that oxidation of the alloy leads, first, to the formation of a surface oxide consisting of UO_2.0. The formation of this oxide was followed by the development of Nb-oxides (NbO, NbO₂ and Nb₂O₅) at the metal-oxide interface. Pure uranium was found to exhibit faster oxidation kinetics than the alloy, while pure niobium oxidised at a much lower rate than the uranium based materials.     

Surface oxidation investigation of Ni36Fe32Cr14P12B6 glass using angle resolved XPS

Native oxide and in-situ prepared, dry oxides of Ni₃₆Fe₃₂Cr₁₄P₁₂B₆ metallic glass have been investigated using angle resolved X-ray photoelectron spectroscopy (XPS or ESCA). The core-level binding energies of the various constituents of clean and oxidized samples have been determined accurately. A qualitative as well as quantitative estimation of elements in the outermost surface layers with and without oxidation is given by comparing XPS results obtained at normal and grazing emission angles. Stepwise oxidation leads to growing thickness of the surface oxide layer and to identification of different oxide species. The maximum thickness of the in-situ prepared oxide was determined as 3.5 nm compared to 4.5 nm for the native oxide. The sequence of oxidation is found to be Cr, Fe, B, P and Ni, but only some of the P and Ni atoms in the surface region are oxidized. The oxidation reaction induces diffusion of the constituents in the surface region as monitored by the change of relative intensities of the various peaks. For instance, P and especially Ni are strongly depleted in the oxide layer whereas Fe, Cr, and especially B are enriched. Differences between native and dry oxide have been observed and are discussed. The main difference is the abundance of carbon and oxygen containing species other than oxides in the native layer. Ar⁺ sputtering of the dry oxide layer leads to stochiometric changes in the surface region which are due to preferential sputtering.     

Electron-stimulated desorption of europium atoms from an oxidized tungsten surface

The yield of europium atoms in electron-stimulated desorption from Eu layers adsorbed on the surface of oxidized tungsten was studied with a surface-ionization detector as a function of the incident-electron energy, surface coverage by europium, and degree of tungsten oxidation. The yield of Eu atoms measured as a function of electron energy exhibits a distinct resonant character with peaks at electron energies corresponding to europium and tungsten core-level ionization energies. The peaks associated with the europium ionization reach a maximum intensity at europium coverages less than 0.1 and decrease subsequently to zero with increasing coverage, while the peaks due to tungsten ionization pass through the maximum intensity at a monolayer europium coverage. The coverage corresponding to the maximum europium atom yield grows with increasing tungsten oxidation. The results obtained are accounted for by the formation of the europium and tungsten core excitons. In the first case, the particles desorb in the reverse motion toward the surface of the oxidized tungsten; in the second, they desorb as a result of repulsion between the tungsten core exciton and the EuO molecule.     

Untersuchung über den einfluss von adsorbierten gasen auf die elektronen-energieverlust-spektren einer Ni(110)-oberfläche

Electron energy loss spectra on a (110) nickel surface exhibit characteristic changes upon adsorption of H₂, CO and O₂. The clean surface shows only the surface and bulk plasmon losses at 8 eV and 18 eV respectively. Adsorption of CO produces two new loss peaks at 13.5 eV and 5.5 eV. Loss peaks due to hydrogen adsorption at 15 eV and 7.5 eV show a strong correlation with the well known adsorption characteristics of this system. The oxygen induced losses are different for chemisorbed O on Ni and NiO. In any case the chemisorption-induced losses are well established for primary energies below 120eV. In the loss spectra with higher excitation energies only a drastic decrease of the surface plasmon loss peak-height is visible. If the new losses can be attributed to one-electron excitations from molecular orbital levels due to the chemisorption bond, with assumptions of the final state of the excited electron a determination of the postition of these levels can be made. In case of CO and H₂ reasonable results are evaluated.     

Assessment of the Nd/U ratio for the quantification of neodymium in UO2

The secondary ion energy spectrum of uranium depends on the experimental conditions, which is a problem for the application of a relative sensitivity factor (RSF) procedure for the measurement of the Nd concentration in UO₂. This is interpreted as due to the dependence of the sputtering and ionisation yields of uranium on the oxidation state of the surface. Three experimental procedures are being tested to cope with this difficulty.     

A thermal desorption study of thiophene adsorbed on the clean and sulfided Mo(100) crystal face

The adsorption of thiophene (C₄H₄S) on the clean and sulfided Mo(100) crystal surface has been studied. A fraction of the adsorbed thiophene desorbs molecularly while the remainder decomposes upon heating, evolving H₂ and leaving carbon and sulfur deposits on the surface. The reversibly adsorbed thiophene exhibits three distinct desorption peaks at 360, 230–290 and 163–174 K, corresponding to binding energies of 22, 13–16 and 7–9 kcal/mol respectively. Sulfur on the Mo(100) surface preferentially blocks the highest energy binding state and causes an increase in the amount of thiophene bound in the low binding energy, multilayer state. The thiophene decomposition reactions yield H₂ desorption peaks in the temperature range 300–700 K. We estimate that 50–66% of the thiophene adsorbed to the clean Mo(100) decomposes. The decomposition reaction is blocked by the presence of c(2 × 2) islands of sulfur and is blocked completely at θ_s = 0.5, at which point thiophene adsorption is entirely reversible.     

A synchrotron XPS study of the vanadia–titania system as a model for monolayer oxide catalysts

The deposition of vanadium metal onto stoichiometric TiO₂(110) has been studied with photoelectron spectroscopy from low to high coverages of vanadium. A synchrotron source was employed in XPS experiments for the study of submonolayer coverages of vanadium in order to determine the oxidation state of the vanadia species formed at submonolayer coverages. The exposure of the titania surface to vanadium metal results in charge-transfer from vanadium to titania at the interface. At low doses of the metal vigorous interaction between the metal and titania surface yields reduction of the Ti⁴⁺ species to Ti³⁺ at the interface, as evidenced by both changes in the lineshape of the Ti 2p XPS spectra and Ti LIII-edge spectra. Concurrent with this surface reduction vanadium metal is oxidized. At higher vanadium doses the vanadium 2p binding energy indicates the formation of metallic vanadium. When metallic vanadium deposition is followed by exposure of the surface to oxygen, only one vanadium species remains on the surface, the binding energy of which corresponds to that of the oxide present initially at low doses of vanadium metal. By comparison of the V 2p binding energies to those of bulk oxides, it appears that the oxidation state of the vanadium in the oxide species is +3, suggesting the formation of V₂O₃ on the surface. Vanadium LIII-edge data also suggest that V₂O₅ is not formed by the oxidation of predosed vanadium metal.     

binding states of CO and H2 on clean and oxidized (111)Pt

The binding states and sticking coefficients of CO and H₂ on clean and oxide covered (111)Pt are examined using flash desorption mass spectrometry and Auger electron spectroscopy (AES). On the clean surface at 78 K there is one major binding state of CO with a desorption activation energy which decreases with coverage plus a second smaller state, while H₂ exhibits three binding states with peak temperatures of 140, 230 and 310 K and saturation density ratios of 0.5 : 1 : 1. Desorption kinetics of CO are consistent with a first order state with a normal pre-exponential factor of 10^{13 ± 1} sec^?1, while all three peaks of H₂ are broader than expected. Interpretations in terms of anomalous pre-exponential factors, coverage dependent desorption activation energies, and desorption orders are considered. On the oxidized surface saturation densities of both gases are nearly identical to those on the clean surface, but desorption temperatures are increased significantly and the initial sticking coefficient on the oxide decreases slightly for CO and increases slightly for H₂.     

Ionic implantation of N2+ in Ni(110) at 300 K

The present work gives results of a preliminary investigation, carried out by SES, AES, LEED and ELS, on the implantation of nitrogen ions in Ni(110) as a function of ion dose and subsequent surface heat treatment at different temperatures. The fine structure in the SES spectrum is the most sensitive to implantation: peaks at 9, 17.5 and 31.5 eV are shifted towards lower energies by E = 1 eV for the first two peaks and 2.8 eV for the last. At high nitrogen doses a disordered layer is observed by LEED. The p(2 × 3) structure is obtained when the crystal is heated to 750 K. The two electron loss peaks of 4.8 and 10 eV arise from an induced electron N_2p level situated 4.8 eV below the Fermi level.     

Electron-stimulated desorption of rare-earth metal atoms

The yield of europium and samarium atoms in electron-stimulated desorption from layers of rare-earth metals (REMs) adsorbed on the surface of oxidized tungsten has been measured as a function of the incident electron energy, surface coverage by REMs, degree of tungsten oxidation, and substrate temperature. The measurements were performed using the time-of-flight method with a surface-ionization-based detector within the substrate temperature interval 140–600 K. The yield studied as a function of electron energy has a resonance character. Overlapping resonance peaks of Sm atoms are observed at electron energies of 34 and 46 eV, and those of Eu atoms, at 36 and 41 eV. These energies correlate well with the REM 5p and 5s core-level excitation energies. The REM yield is a complex function of the REM coverage and substrate temperature. The peaks due to REM atoms are seen at low REM coverages only, and their intensity usually passes through a maximum with increasing coverage and substrate temperature. The concentration dependence of the REM atom yield is affected by the deposition of slow Ba⁺ ions, but only if they are deposited after the REM adsorption. At higher REM coverages, additional peaks are observed at electron energies of 42, 54, and 84 eV, which originate from excitation of the 5p and 5s tungsten levels and result from desorption of SmO and EuO molecules. The temperature dependence of the intensity of these peaks is explained to be due to the order-disorder phase transition. The desorption of REM atoms is the result of their reversed motion through the adsorbed REM layer, and the SmO and EuO molecules desorb due to the formation of an antibonding state between the REM oxide molecules and the tungsten ions.     

XPS study of SnTe(1 0 0) oxidation by molecular oxygen

The interaction between the (1 0 0) surface of SnTe single crystal and molecular oxygen was studied by means of X-ray photoelectron spectroscopy (XPS). Analysis of the obtained spectra shows that the mechanism of surface oxidation does not change in the range of oxygen exposure 10⁸-10¹³ L. During the oxidation an additional component shifted 1.1 eV towards higher binding energies appears in the Sn 3d spectra. The Te 3d_5/2 spectra fitting reveals two additional components with binding energies close to Te⁰ and Te⁺⁴. The dependence of the additional components fraction in both Sn 3d and Te 3d_5/2 spectra on the oxygen exposure is semi-logarithmic. On the base of the experimental data two possible mechanisms are proposed.     

Devaluation of the gold standard in x-ray photoelectron spectroscopy

The value of vapor-deposited gold as a standard in x-ray photo-electron spectroscopy has been studied. During the deposition gold may react with the sample giving consequent shifts and/or broadening of the Au 4f peaks. Shifts to higher binding energies have been observed with KCN and NACl and to lower energies with Na₂S₂O₃ and copper phthalocyanine. The An 4f peaks have been investigated over the temperature range ?50 to 200°C. At higher temperatures peaks due to metallic gold are observed. Anomalous effects have been observed in the XPS spectrum of gold plated copper phthalocyanine in the binding energy range 650–950 eV.     

Energy levels of hydrogenic impurity states in GaAs-Ga1−xAlxAs quantum well structures

Binding energies of the ground state and of four excited states of a hydrogenic impurity in quantum well structures consisting of a single slab of GaAs sandwiched between two semi-infinite slabs of Ga_1?xAl_xAs are calculated using a variational approach. The ground-state binding energy is calculated as a function of the barrier potential for a given size of the GaAs quantum well and is found to be linearly dependent on the inverse of the square root of the barrier potential except for very small potentials. The variation of the binding energies of all five states as a function of the size of the GaAs quantum well are also calculated and their behavior is discussed.     

X-ray diffraction and photoelectron spectroscopy studies of MoO2 as catalyst for the partial oxidation of isooctane

X-ray diffraction (XRD), X-ray photoemission (XPS) as well as ultraviolet photoemission (UPS) spectroscopy experiments on MoO₂ powders were carried out to examine the bulk, the core level energies, and the electronic structure of MoO₂ samples that were employed as catalysts for the partial oxidation of isooctane. Five fresh 0.5-g MoO₂ samples were exposed for 0, 0.5, 9, 20, and 43 h to identical reforming environments and their spent samples were analyzed using the techniques mentioned above. Our results indicate the rapid appearance of an intermediate Mo phase with a binding energy of 228.5 eV and whose concentration progressively increases with time. The oxidation state for this new phase was graphically estimated to approximately +2.6 and assigned to the compound Mo₂O₃, which forms on the catalyst surface as a result of its exposure to the reforming environment. The electronic structure probed by UPS reveals two bands, one at 1.62 eV and another at 0.55 eV below the Fermi level, that decrease with the increasing time on stream. These results correlate very well with the drop in the catalytic performance of MoO₂ for the partial oxidation of isooctane and with the decline in the concentration of dioxide (Mo⁴⁺) detected not only on the catalyst surface, but also in the bulk structure, as confirmed by our XRD analysis.     

Crystallization kinetics in phosphate sodium-based glass studied by DSC technique

In this work, differential scanning calorimetry (DSC) was used to study the mechanism of crystallization of 50P₂O₅-27.8Na₂O-16ZnO-6.2Al₂O₃ glass. DSC measurements were performed on bulk and powdered glasses with different particle size. The curve for bulk glass shows one crystallization peak while powdered glasses presented two distinct crystallization peaks. Based on DSC studies, the activation energies obtained were 336±6 and 213±3 kJ mol⁻¹, associated with first and second crystallization peaks, respectively. Analyzing the DSC parameters as a function of particle size, the Avrami n parameter suggests that the peak at low temperature may be associated with surface crystallization while the peak at high temperature is associated with bulk crystallization.     

磁控溅射沉积铝/贫铀与金/贫铀镀层的界面研究

采用磁控溅射技术沉积制铝/贫铀/铝(Al/DU/Al)、金/贫铀/金(Au/DU/Au) "三明治" 薄膜样品. 利用高分辨扫描电镜、 X射线衍射仪、X射线光电子能谱仪、 扫描俄歇微探针对Al/DU/Al, Au/DU/Au样品的Al/DU, Au/DU界面行为进行表征与研究. 结果表明: 沉积态DU层以柱状晶生长; Al/DU界面扩散明显, 物理扩散过程中伴随着Al, DU化学反应形成Al₂U, Al₃U金属化合物; 金属化合物的形成导致界面处Al 2p电子结合能向高能端移动, U 4f电子向低能端移动; 微量O在Al/DU界面处以Al₂O₃及铀氧化物形式存在; DU镀层中以铀氧化形式存在; 沉积态的Au/DU界面扩散为简单的物理扩散, 团簇效应导致Au/DU界面处Al 2p, U 4f电子结合能均向高能端移动; 在Au/DU界面及DU镀层中, 微量O以铀氧化物形式存在; Al/DU界面扩散强于Au/DU; 相同厚度的Al, Au保护镀层, Al镀层保护效果优于Au镀层. 关键词： Al/DU界面 Au/DU界面 磁控溅射 界面扩散     

Characterisation of thin films of bismuth oxide by X-ray photoelectron spectroscopy

The XPS (or ESCA) technique has been used for the characterisation of vacuum-deposited thin films of bismuth oxide. The spectra of Bi metal and Bi₂O₃ powder are used for comparison. The characterisation is carried out by consideration of the positions of the Bi 4f_{$\text{7}\text{2}$} and 4f_{$\text{5}\text{2}$} peaks and by using peak-fitting routines. A lower suboxide of bismuth, and metallic bismuth are observed in bismuth oxide films as evaporated. Oxidation of these films by heating in air results in bismuth(III) oxide. A linear relation is found between the binding energies and oxidation state. The corresponding O 1s spectra for the two types of film are also discussed.