Reaction of D₃O⁺ with D₂: Proton affinity of water

The cross section for the endothermic ion-molecule reaction: D3O ⁺(D₂D₂O)D₃O⁺ has been studied over the energy range 2-70 eV using a tandem mass spectrometer. The projectile beam was found to contain a significant fraction of the D ₃O⁺ in an excited electronic state which was relatively insensitive to collisional deactivation using high pressures but was readily deactivated using the paramagnetic gas NO in the primary ion source. Threshold measurements, using the ground state D3O⁺ beam, permitted a determination of the proton affinity of water and the conclusion that the reaction proceeded through a persistent intermediate complex at energies close to the threshold.