Synthesis of dioxygen in a CO2 matrix: Positive ion desorption at the solid ionisation threshold

O2+ ion desorption as a reaction product from solid CO2 ice has been observed for the first time. The desorption threshold is at the ionisation potential for solid CO2, ∼6 eV below that for O2+ production in the gas phase. The threshold for CO+ ions is also substantially below the gas phase threshold. No positive ion desorption has been seen at or below the solid ionisation threshold in any condensed gas before. Both solid threshold energies are a result of multielectron excitations.