Electronic excitation of uranium atoms sputtered from uranium metal and oxides

Abstract

Atoms sputtered from solids by energetic ion irradiation often partition across electronically excited states. We developed a resonance ionization method to measure the population of the uranium ⁵K₅ state (620 cm⁻¹) relative to the ⁶L₆ ground state for atoms sputtered from metallic and oxide surfaces. A considerable fraction of sputtered atoms reside in the ⁵K₅ state, and the fraction increases as the surface oxidizes. The effect is consistent across uranium metal, UO₂, and U₃O₈, and is invariant with the departing velocity of the sputtered atom. The data are not sufficient to determine the excitation mechanism, however they are qualitatively consistent with resonant electron transfer from the surface to the departing atom.