RIMS analysis of ion induced fragmentation of molecules sputtered from an enriched U3O8 matrix

Resonance ionization mass spectrometry was used to measure the composition of the sputtered flux from 15 keV Ga⁺, Au⁺, Au₂ ⁺ and Au₃ ⁺ primary ions impacting a ²³⁵U enriched U₃O₈ standard. We demonstrate that molecular fragmentation decreases as the primary ion mass and nuclearity increases. Stopping and range of ions in matter calculations show that cluster ions (Au₂ ⁺ and Au₃ ⁺) deposit more of their energy via direct knock-ons with near-surface target atoms, whereas monatomic ions (Ga⁺ and Au⁺) penetrate much deeper into the target sub-surface region. We correlate these results to the experimental observations by showing that increased cluster ion sputter yields partition the projectile energy over a larger number of sputtered molecules. Therefore, while cluster ions deposit more total energy into the near surface region of the target compared to monatomic ions, the energy per molecule decreases with projectile mass and nuclearity. Less energy per molecule decreases the number of U–O bond breaks and, consequently, leads to a decrease in molecular fragmentation. Additionally, the extent of molecular fragmentation as a function of ion dose was evaluated. We show that molecular fragmentation increases with increased ion dose; primarily as a result of sub-surface chemical damage accumulation. The relative intensity of this effect appears to be projectile independent.