Synthesis and investigation of plutonium oxide cluster ions: Pu xO y+ ( x≤18)

Abstract

The plutonium–oxygen system is fundamentally and technologically important. The primary goal of the present study was to prepare and carry out initial chemical studies on gas-phase plutonium oxide cluster ions, which would contribute to the general understanding of PuO x chemistry. In this effort, several plutonium oxide, oxide-hydroxide and hydroxide cluster ions, Pu x O y (OH) z +, were synthesized by pulsed laser ablation of hydrated plutonium oxalate in vacuum. Cerium was studied for comparison, and a chemically less diversified oxide cluster system was observed. The large variety of Pu x O y (OH) z + compositions found is attributed to the accessibility of multiple oxidation states for Pu, and the results demonstrate the feasibility of preparing clusters with diverse stoichiometries and variable plutonium oxidation states. These clusters may be studied by various gas-phase techniques to illuminate the behavior of plutonium as a function of key variables, such as oxidation state, structure/coordination and bonding. In addition, some gas-phase reaction studies were carried out and at least one significant cluster reaction product with dimethylether was identified. This result demonstrates the desirability of studying plutonium cluster chemistry in ion traps where longer residence times, and more ion–molecule collisions will result in greater reaction efficiencies. The largest primary oxide cluster identified here comprised six Pu atoms, but two distinctive ‘magic number’ clusters were also produced, which incorporated 16 and 18 Pu atoms, respectively. These ‘magic number’ clusters suggest that special stabilities obtain for these species and that it may be possible to prepare plutonium oxide ‘nanocrystals’ in the gas phase for examination by techniques complementary to those applicable to bulk materials. The plutonium cluster compositions and abundances observed in this work are discussed in the context of the oxidation states, structures, and bonding exhibited by plutonium.