Reaction dynamics studies of a simple tetraatomic system: AA + BA ↔ A + ABA

Abstract

We have studied the reaction of translationally excited hydroxyl radicals with molecular hydrogen at different center-of-mass energies. H/D atoms produced in the reaction have been detected under single collision conditions by means of vacuum ultraviolet laser-induced fluorescence at the Lyman-α transition. By calibrating the H/D signals from the reaction against H atom signals from the H 2 S and HCl photolysis, respectively, the following absolute reactive cross sections were determined: OH + H 2 → H + H 2 O: σ R (0.17 eV) = (0.08 ± 0.03) Å 2 , σ R (0.22 eV) = (0.60 ± 0.30) Å 2 ; OH + D 2 → D + HOD: σ R (0.28 eV) = (0.22 ± 0.05) Å, σ R (0.37 eV) = (0.43 ± 0.09) Å 2 . With translationally excited H atoms, we have also studied the reverse reaction system, H + D 2 O. In this case the OD product radicals were detected under single collision conditions with quantum state resolution by means of laser-induced fluorescence. By calibrating the OD signals from the reaction against OH signals from the H 2 O 2 photolysis, absolute reaction cross sections were measured for H + D 2 O → OD + HD: σ R (1.5 eV) = (0.07 ± 0.04) Å 2 , σ R (1.8 eV) = (0.10 ± 0.03) Å 2 , and σ R (2.2 eV) = (0.11 ± 0.03) Å 2 . At different center-of-mass collision energies nascent population distributions of the OD product fine-structure components were determined. It has been found that at all collision energies OD radicals are produced exclusively in their vibrational ground state, with only a small amount of the total available energy appearing in the rotational degree of freedom. A comparison of the dependence of the reaction cross section on the translational energy shows that relative reagent translation is more effective to promote reactivity in the reaction OH + H 2 and OH + D 2 than in the reaction H + D 2 O. This, together with a preferential population of the symmetric 2 Π(A′) Λ-doublet state (with the unpaired π orbital lying in the plane of rotation) at high OD rotational quantum numbers, suggests that the reaction H + D 2 O → OD + HD proceeds through a planar transition state via a direct mechanism, where the OD moiety acts as a spectator, and where the reaction barrier is located at a "later" position of the reaction coordinate.