Electronic Structure and Surface Science of delta Plutonium
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Electronic Structure and Surface Science of delta Plutonium M. Butterfield1; T. Durakiewicz1; E. Guziewicz1; J.J. Joyce1; D.P. Moore2; A.J. Arko1; L.A. Morales2 1. MST-10, LANL, Los Alamos, NM, USA. 2. NMT-16, LANL, Los Alamos, NM, USA
Abstract High resolution photoelectron spectroscopy (PES) studies were conducted on a δphase Plutonium sample cleaned by laser ablation and gas dosed with O2 and H2. The measurements were made with an instrument resolution of 60 meV and with the sample at 77 K. The PES data strongly support a model with Pu2O3 growth on the metal and then PuO2 growth on the Pu2O3 layer at this temperature. In vacuum, the PuO2 reduces to Pu2O3 at room temperature with a pressure of 6x10-11 Torr. In the case of H2 dosing the hydrogen appears to penetrate the surface and disrupt the valence band as evidenced by a drop in intensity of the peak at EF which is not accompanied by a drop in the main 5f manifold at ~2eV.
Introduction The investigation of the actinides is a subject of great interest because of their unique electronic structure [1, 2]. At the pivotal point of the behavior of the electronic structure of the actinide series is Plutonium. The phase diagram of plutonium (Pu) is extremely complex [1]. In fact, it has the most complex phase diagram of all metals, both with regard to the intricacy of the crystal structures and the number of different phases. Of the different allotropes of Pu, the δ- phase is of particular interest because of the high symmetry crystal structure and the stability of the phase to low temperatures by alloying with small amounts of trivalent elements. Consequently much of the recent experimental and theoretical work has focused on this allotrope. From an experimental point of view, the reactivity and radioactivity of Pu, and the complexity of the phase diagram, makes it exceedingly complicated to collect high-quality data. We have already obtained and reported high quality photoemission data for the δ- phase of Plutonium [3] in which we obtained a very clean δ-Pu sample surface using laser ablation. We will present gas dosing studies taken using the same high quality sample. Previous studies of the initial oxidation of Pu [4-9] have found limited degrees of success in obtaining clean sample surfaces free from contamination as a starting point. Almedia and coworkers [10] carried out an excellent study of gas adsorption but their method of surface preparation may have caused a depletion of the Gallium content in the surface region (a small percentage, typically 1-2 weight percent, of Ga is used to stabilize
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the δ- phase at room temperature) which may have lead to the surface consisting of a mix of δ - and α-phases. Plutonium metal surfaces are covered by layers of plutonium oxide, and the surface oxide controls chemical reactivity. Some models for the oxide growth and behavior for the Pu surface are shown in Figure 1. At room temperature (a) the metal is covered by a thin layer of Pu2O3 followed by a layer of PuO2. The layer thickness is sensitive to envi
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