The kinetics of free radicals generated by IR laser photolysis. III. Intersystem crossing between C2(X 1Σg+) and C2(a 3Πu) induced by collisions with oxygen

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Abstract
Reactive and energy transfer processes involving collisions of C2(X 1Σg+) and C2(a 3Πu) with molecular oxygen are reported. Intersystem crossing between C2(X 1Σg+) and C2(a 3Πu) is induced very efficiently by collisions with O2. The triplet→singlet rate coefficient is (2.7+0.3−0.6)×10−11 cm3molecule−1 s−1 and the ratio between this and the singlet→triplet rate coefficient is ∼3 at 300 K. The rate coefficient for reaction is much less than that for intersystem crossing, making the measurement of separate singlet and triplet reaction rate coefficients with oxygen impossible. The previously measured reaction rate coefficient (3×10−12 cm3molecule−1 s−1 at 300 K) for both singlet and triplet C2 removal by O2 is reinterpreted as the rate coefficient for removal of equilibrated C2 molecules. Using hydrocarbon scavengers, it is shown that the source of the observed emissions from excited triplet states of CO is mainly reactions of C2(X 1Σg+) with O2.