Direct observation of pure pentavalent uranium in U2O5 thin films by high resolution photoemission spectroscopy.

Thin films of the elusive intermediate uranium oxide U2O5 have been prepared by exposing UO3 precursor multilayers to atomic hydrogen. Electron photoemission spectra measured about the uranium 4f core-level doublet contain sharp satellites separated by 7.9(1) eV from the 4f main lines, whilst satellites characteristics of the U(IV) and U(VI) oxidation states, expected respectively at 6.9(1) and 9.7(1) eV from the main 4f lines, are absent. This shows that uranium ions in the films are in a pure pentavalent oxidation state, in contrast to previous investigations of binary oxides claiming that U(V) occurs only as a metastable intermediate state coexisting with U(IV) and U(VI) species. The ratio between the 5f valence band and 4f core-level uranium photoemission intensities decreases by about 50% from UO2 to U2O5, which is consistent with the 5f 2 (UO2) and 5f 1 (U2O5) electronic configurations of the initial state. Our studies conclusively establish the stability of uranium pentoxide.

Photoelectron spectroscopy of NpPd(3) and PuPd(3).

We present the results of x-ray and ultraviolet photoelectron spectroscopy of NpPd(3) and PuPd(3). The spectra indicate that for both compounds, the 5f electrons are well localized on the actinide sites. Comparison with bulk measurements indicates that for NpPd(3) the electrons have a valence of Np(3+) and thus a ground state 5f(4) with a Hund's rules (5)I(4) configuration. Similarly for PuPd(3), we find a Pu(3+) valence, 5f(5) ground state and a Hund's rules (6)H(5/2) configuration.

Adsorption and reaction of SO2 with a polycrystalline UO2 film: promotion of S-O bond cleavage by creation of O-defects and Na or Ca coadsorption.

To characterize UO(2) for its possible use in desulfurization applications, the interactions of molecular sulfur dioxide (SO(2)) with a polycrystalline uranium dioxide film have been studied by means of X-ray photoelectron spectroscopy (XPS), temperature-programmed desorption (TPD), and low-energy ion scattering (LEIS). The stoichiometric, oxygen-deficient, calcium-precovered and sodium-precovered UO(2) surfaces have been characterized. The changes in oxide reactivity upon creation of oxygen vacancies and coadsorption of sodium and calcium have been studied. After creation of a reduced UO(2-x) surface (x approximately 0.44) via Ar(+) sputtering, the U 4f XPS spectrum shows conspicuous differences that are good indicators of the surface stoichiometry. Molecular SO(x) formation (x = 2-4) is observed after SO(2) deposition onto stoichiometric UO(2) and onto UO(2) precovered with small amounts (<1 ML) of Na or Ca; complete dissociation of SO(2) is not observed. Heating leads to desorption of the SO(x) species and to transformation of SO(2) to SO(3) and SO(3) to SO(4). On oxygen-deficient UO(2) and on UO(2) precovered with large Na or Ca coverages (> or =4 ML), both the formation of SO(x)= species and complete dissociation of SO(2) are observed. A higher thermal stability of the sulfur components is observed on these surfaces. In all cases for which dissociation occurs, the XPS peak of atomic sulfur shows similar structure: three different binding states are observed. The reactivity of oxygen-deficient UO(2) and sodium- and calcium-precovered UO(2) (coverages > or = 4 ML) is attributed to charge transfer into the antibonding LUMO of the adsorbed molecule.

5f-electron localization in PuSe and PuSb

Thin layers of PuSb and PuSe were studied by photoelectron spectroscopy. X-ray photoelectron spectroscopy and high-resolution valence-band ultraviolet photoelectron spectroscopy spectra show localization of the 5f states and a low density of states at E(F) in PuSb. In PuSe, which can be classified as a heavy fermion system with low carrier density, we observed three narrow peaks in the valence band, which can be related to the 5f emission. These three features are very sensitive to stoichiometry deviations and disappear for PuSe prepared at T = 77 K.