Gas-phase reactivity of actinide monocations with O₂ and CO₂ from thorium to californium

The gas phase reactivity of actinide monocations (Th⁺, Pa⁺, U⁺, Np⁺, Pu⁺, Am⁺, Cm⁺ and Cf⁺) with O₂ and CO₂ was studied using inductively coupled plasma mass spectrometry (ICP-MS). Total reactions were observed for all actinides, except californium, for which only partial reactions occurred. Depending on the actinide, different reaction products are formed. AnO⁺ and AnO₂⁺ products are formed sequentially for Th, Pa, U, Np and Pu, whereas only AnO⁺ is formed for Am, Cm and Cf. Among all investigated actinides, Cf⁺ is the least reactive. The results are compared to experimental and calculated bond dissociation energies of the AnO₂⁺ and AnO⁺ products. The smallest bond dissociation energies are found for AmO₂⁺, CmO₂⁺ and CfO₂⁺, suggesting that these species are the least stable, as expected given their lack of formation. Among the actinide monoxides, the AmO⁺ and CfO⁺ species exhibit the lowest bond dissociation energy, consistent with their weaker reactivity toward O₂ and CO₂ compared with the other actinides.