Auto-ionised products from the reaction of sodium clusters with dioxygen: Theory and experiment

Abstract

The reactive collision between sodium clusters and dioxygen is investigated by crossed beam experiment and density functional calculations. Specific cations, particularly Na₅O₂⁺, are detected in the auto-ionisation mass spectrum, which contrasts with the less selective range of neutral products detected in the photoionisation mass spectrum. This is rationalised with the aid of ab initio molecular dynamics and the computed energies of fragmentation of dioxides Na_nO₂⁽⁺⁾ (n⩽9) to clusters of monoxides Na_nO⁽⁺⁾ and pure sodium Na_n⁽⁺⁾. A redox sequence is observed in simulation from superoxide to peroxide to bis-monoxide (O₂)⁻→(O₂)²⁻→2O²⁻, which releases enough energy to allow fragmentation and auto-ionisation. Exoelectron emission may therefore be ultimately connected with the reductive dissociation of the O–O bond. Some confidence in the underlying hypersurface is obtained from comparison with experimental and higher-level theoretical data, including structures and energetic properties (previously published), and the topology of the Na₂O₂ superoxide–peroxide electron transfer curve.