Model atomic oxygen reactions: Detailed experimental and theoretical studies of the reactions of ground-state O(³P) with H₂, CH₄, CH₃CH₃, and CH₃CH ₂CH₃ at hyperthermal collision energies

The reactions of O(³P) with H₂, CH₄, CH₃CH₃, and CH₃CH₂CH₃ at center-of-mass collision energies in the range 1.5-3.9 eV have been investigated with (1) crossed-beams experiments employing a laser-detonation source of O atoms and (2) various theoretical methods. We present here the first measurements of the relative excitation function for the O(³P) + H₂ reaction, and the experimental results are in excellent agreement with accurate quantum wave packet calculations. The experiment-theory agreement confirms that the laser-detonation source produces oxygen atoms in the ground O(³P) state. For reactions of O(³P) with alkanes, the experiments provide evidence for previously unobserved reaction pathways which principally lead to O-atom addition and subsequent H-atom elimination or C-C bond breakage: O(³P) + RH - RO + H or R'O + R″. In addition, the expected H-atom abstraction reaction to form OH has been observed. The H-atom abstraction reactions have modest barriers in the range ∼0.1 - 0.3 eV, whereas the addition pathways have barriers greater than ∼1.8 eV. Nevertheless, theory predicts that abstraction and addition reactions occur with similar probabilities at the collision energies of these studies. We have thus far completed one in-depth study of such an addition reaction: O(³P) + CH₄ - OCH₃ + H. Although high barriers prevent the addition reactions from occurring in most thermal environments, such reactions might be important in low-Earth orbit, where spacecraft surfaces and exhaust gases suffer high-energy collisions with ambient atomic oxygen.