American Chemical Society
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Dynamics of the O‑Atom Exchange Reaction 16O(3P) + 18O18O(3Σg) → 16O18O(3Σg) + 18O(3P) at Hyperthermal Energies

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posted on 2016-04-04, 00:00 authored by Sridhar A. Lahankar, Jianming Zhang, Timothy K. Minton, Hua Guo, György Lendvay
The O atom exchange reaction, 16O­(3P) + 18O18O­(3Σg) → 16O18O­(3Σg) + 18O­(3P), was investigated at a hyperthermal center-of-mass (c.m.) collision energy (Ecoll) of 86 kcal mol–1, using a crossed-molecular-beams apparatus and quasiclassical trajectory (QCT) calculations. The inelastically scattered 16O and reactively scattered 16O18O products were detected with a rotatable mass spectrometer employing electron-impact ionization. The 16O atoms are scattered in inelastic collisions in the forward direction relative to their initial direction of flight, with most of the available energy partitioned into translation. The 16O18O products of reactive collisions are mainly formed through impulsive dynamics and are scattered in the forward as well as sideways directions relative to the direction of the reagent 16O atoms, with a slight majority of the available energy partitioned into translation (⟨ET⟩ = 58%) and a significant contribution to internal degrees of freedom. Excellent agreement was found between the experimental c.m. angular and translational energy distributions of the inelastically scattered 16O and reactively scattered 16O18O products and those obtained from QCT calculations, which were carried out on a ground-state singlet electronic potential energy surface. The QCT calculations predicted 16O18O products that are both highly rotationally and vibrationally excited, with j′(16O18O) up to 150 and v′(16O18O) up to 15, respectively. The QCT simulations indicate that the translational energy distribution of the reactively scattered 16O18O is bimodal, corresponding to two distinct interaction mechanisms that are dependent on impact parameter: one at impact parameters below ∼0.5 Å and another in the vicinity of 1.6 Å. Collisions in the former regime produce 16O18O with internal energy closer to the maximum available energy while the latter mechanism, involving strong interaction within the O3 potential well, is responsible for the low-energy peak of the product translational distribution. The inelastic collisions also follow two basic impact-parameter-dependent mechanisms. At impact parameters above 2.1 Å, the 16O atom is reflected from the outer repulsive wall of the O2 molecule, resulting in exclusively forward scattering, while collisions at impact parameters below ∼2 Å access the O3 potential well and lead to ejection of either an 16O or an 18O atom. Scattering remains preferentially forward in both cases due to the large momentum of the attacking 16O atom.