Configuration interaction study of the O₂-C₂H₄ exciplex: Collision-induced probabilities of spin-forbidden radiative and non-radiative transitions

The Collision complex between molecular oxygen and ethene has been studied by configuration interaction calculations, scanning the intermolecular distance in C_2v collision symmetry. The calculations give a good explanation for the enhancement of the ethene T₁ ← S₀ and the oxygen A′³Δ_u ← X³Σ_g^- transition probabilities observed by Evans at high oxygen pressure. A number of other oxygen and cooperative transitions are explained and predicted, considering the studied system as a general model for unsaturated hydrocarbon-O₂ interactions. The intermolecular potentials show negligible complex formation energies for the lower X³Σ_g^-, a ¹Δ_g and b ¹Σ_g⁺ states, but exciplex character for higher states. The ethene (ππ*) excited S₁ state has a fairly deep minimum and a very strong transition moment to the T₁ state, which also accounts for the non-radiative quenching of the hydrocarbon fluorescence by molecular oxygen.