Hyperthermal reactions of O(p3sP) with hydrogen and methane
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Date
2004
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Montana State University - Bozeman, College of Letters & Science
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.