O2 adsorption on UNi5 studied by ion scattering spectroscopy and UV photoelectron spectroscopy

Abstract

Ion scattering spectroscopy (ISS) was used to study the adsorption of O2 on UNi5 at room temperature. The main goal was to follow the changes of the stoichiometry of the surface with O2 exposure exploiting the topmost surface layer sensitivity of ISS. Simultaneous ultraviolet photoemission spectroscopy (UPS/Hell, 40.81 eV) analysis allowed to determine related changes of the valence electrons which are responsible for the chemical reactivity of the surface. The ISS/He+ data show that at an exposure of L ≈ 3 langmuir of O2 no Ni atoms are detected in the topmost surface layer. However, the less surface-sensitive UPS spectra still reveal the presence of Ni in the surface. Submitting the sample to higher exposure ISS shows a He+/O contribution, which increases with the exposure, but a constant He+/U peak intensity. The UPS spectra confirm previous results: the U metal derived 5f peak at the Fermi-edge disappears, the U dioxide derived 5f2 peak increases and the O 2p line is no more symmetric and develops a shoulder at higher binding energy. At high exposure values the surface will be composed of UO2 with Ni metal below the topmost layer. The ISS data suggest that an exponential (e−Lα) will describe the evolution of the U concentration with α ≈ 2.7 langmuir. The UPS suggest that the evolution of the O 2p intensity can also be described by (1−e−Lβ) with β ≈ 2α ≈ 5.4 langmuir.