Hyperthermal reactions of O(p3sP) with hydrogen and methane

Abstract

Hyperthermal reactions of O(3P) occur at the surfaces and in the exhaust gases of spacecraft that travel through the residual atmosphere of the Earth at high altitudes (200-600 km). These reactions may degrade materials through oxidation and erosion, or they may yield internally excited reaction products which emit radiation and contribute to the “signature” of a rocket plume. Crossed-beams experiments were used to study model reactions of O(3P) with H2, D2, CH4, and CD4 at center-of-mass collision energies in the range 8-75 kcal mol^-1. Interpretation of the experimental results has been strengthened by theoretical calculations carried out by collaborators. A study of the OH scattered flux as a function of collision energy has led to the determination of an experimental excitation function in the threshold region for the O(3P)+H2 → OH+H reaction. The experimental excitation function clearly matched the theoretical prediction, which confirmed that the laser-detonation source produces O(3P) atoms. The excitation function for the O(3P) + H2 reaction and the dynamics of the O(3P) + D2 reaction, observed experimentally for the first time, demonstrate that these reactions proceed mainly on triplet potential energy surfaces, with little or no intersystem crossing. Experiments on the reactions of O(3P) with methane have revealed a previously unobserved reaction pathway, which involves H-atom elimination: O(3P) + CH4 → OCH3 + H. The excitation function for this reaction has been measured, and the reaction barrier has been determined to be ∼46 kcal mol^-1. In addition, the expected H-atom abstraction reaction, O(3P) + CH4 → CH3 + OH, has been observed, and the dynamics have been investigated. Theoretical calculations identify a triplet-singlet curve crossing below the triplet barrier for the H-atom elimination reaction, but the observed dynamics indicate reaction exclusively on the two lowest-lying triplet surfaces. While it remains to be seen whether intersystem crossing will affect the outcome of other reactions involving hyperthermal atomic oxygen, unknown reactions which have high barriers are likely to be common in extreme environments such as low-Earth orbit, where spacecraft surfaces and exhaust gases suffer high-energy collisions with ambient atomic oxygen.