Dynamics of the O‑Atom Exchange Reaction ¹⁶O(³<i>P</i>) + ¹⁸O¹⁸O(³Σ_g^–) → ¹⁶O¹⁸O(³Σ_g^–) + ¹⁸O(³<i>P</i>) at Hyperthermal\nEnergies

Abstract

The\nO atom exchange reaction, ¹⁶O­(³<i>P</i>) + ¹⁸O¹⁸O­(³Σ_g^–) → ¹⁶O¹⁸O­(³Σ_g^–) + ¹⁸O­(³<i>P</i>), was investigated at a hyperthermal center-of-mass\n(c.m.) collision energy (<i>E</i>_coll) of 86\nkcal mol^–1, using a crossed-molecular-beams apparatus\nand quasiclassical trajectory (QCT) calculations. The inelastically\nscattered ¹⁶O and reactively scattered ¹⁶O¹⁸O products were detected with a rotatable mass spectrometer\nemploying electron-impact ionization. The ¹⁶O atoms are\nscattered in inelastic collisions in the forward direction relative\nto their initial direction of flight, with most of the available energy\npartitioned into translation. The ¹⁶O¹⁸O products\nof reactive collisions are mainly formed through impulsive dynamics\nand are scattered in the forward as well as sideways directions relative\nto the direction of the reagent ¹⁶O atoms, with a slight\nmajority of the available energy partitioned into translation (⟨<i>E</i>_T⟩ = 58%) and a significant contribution\nto internal degrees of freedom. Excellent agreement was found between\nthe experimental c.m. angular and translational energy distributions\nof the inelastically scattered ¹⁶O and reactively scattered ¹⁶O¹⁸O products and those obtained from QCT calculations,\nwhich were carried out on a ground-state singlet electronic potential\nenergy surface. The QCT calculations predicted ¹⁶O¹⁸O products that are both highly rotationally and vibrationally\nexcited, with <i>j</i>′(¹⁶O¹⁸O) up to 150 and <i>v</i>′(¹⁶O¹⁸O) up to 15, respectively. The QCT simulations indicate that the\ntranslational energy distribution of the reactively scattered ¹⁶O¹⁸O is bimodal, corresponding to two distinct\ninteraction mechanisms that are dependent on impact parameter: one\nat impact parameters below ∼0.5 Å and another in the vicinity\nof 1.6 Å. Collisions in the former regime produce ¹⁶O¹⁸O with internal energy closer to the maximum available\nenergy while the latter mechanism, involving strong interaction within\nthe O₃ potential well, is responsible for the low-energy\npeak of the product translational distribution. The inelastic collisions\nalso follow two basic impact-parameter-dependent mechanisms. At impact\nparameters above 2.1 Å, the ¹⁶O atom is reflected\nfrom the outer repulsive wall of the O₂ molecule, resulting\nin exclusively forward scattering, while collisions at impact parameters\nbelow ∼2 Å access the O₃ potential well and\nlead to ejection of either an ¹⁶O or an ¹⁸O\natom. Scattering remains preferentially forward in both cases due\nto the large momentum of the attacking ¹⁶O atom.