MODEL ATOMIC OXYGEN REACTIONS: DETAILED EXPERIMENTAL AND THEORETICAL STUDIES OF THE REACTIONS OF GROUND-STATE O(~3P) WITH H_2, CH_4, CH_3CH_3, AND CH_3CH_2CH_3 AT HYPERTHERMAL COLLISION ENERGIES

摘要

The reactions of O(~3P) with H_2, CH_4, CH_3CH_3, and CH_3CH_2CH_3 at center-of-mass collision energies in the range 1.5-3.9 eV have been investigated with crossed-beams experiments employing a laser-detonation source of O atoms and various theoretical methods. We present here the first measurements of the relative excitation function for the O(~3P) + H_2 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) + CH_4 → OCH_3 + 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.