Hydrogen and Oxygen Gas Dosing on α-Plutonium Using Time-of-Flight Secondary Ion Mass Spectroscopy and Density Functional Theory

Plutonium metal is highly reactive towards environmental gases by immediately forming an oxide layer when exposed to air and quickly forming a hydride when exposed to hydrogen. Previous work has shown that the monoclinic α-Pu phase readily oxidizes quicker than compared to the face-centered-cubic δ-phase. By using ToF-SIMS in conjunction with DFT calculations, we exposed α-Pu to hydrogen and oxygen gas to determine the reactivity. ToF-SIMS is a highly surface-specific analytic technique that probes <1nm of the surface and can determine impurities at parts-per-million levels and chemical speciation, including hydrogen. After H₂ and O₂ gas exposures of an Ar⁺ sputtered cleaned α-Pu metal, there was no apparent reaction to H₂ for up to 180 Langmuir (L), where at ~20-30L of O₂ exposure oxide formation was evident. This is indicated by the relativity intensities of the negative Pu-O ion fragments being PuO₃^- > PuO₂^- > PuO^- as the surface is approaching a “PuO₂” state. The experimental observations were supported by DFT studies, which indicated that H₂ physisorbs on the α-Pu surface at 0 K independent of coverage and partially dissociates at high surface coverage at 300 K, whereas O₂ strongly reacts to form oxide layers at 0 K at both low and high surface coverage.