Emission of Vacuum Ultraviolet Radiation from the Acetylene-Oxygen and the Methane-Oxygen Reactions in Shock Waves

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Abstract
The C2H2–O2 and CH4–O2 reactions were studied by observing the emission of vacuum ultraviolet radiation from shock waves in gas mixtures containing 85 to 99% Ar and with 2×10−6≤[O2]≤60×10−6 moles/l. In both reactions after an induction period of length ti, the emission intensity rises exponentially with a time constant τ and then decreases about as rapidly. For the C2H2–O2 reaction, emission predominantly in the wavelength range 1500<λ<1700 A was observed. It was estimated that one photon was emitted in this range per 3×104 C2H2 molecules passing through the reaction zone at 1840°K. The integrated emission intensity depends on the temperature as if the process producing it has an activation energy of 15 kcal/mole. In the range of conditions 0.65≤[O2]/[C2H2]≤3.6 and 1300≤T≤2200°K it was found that log10([O2]ti) (mole sec/l)=−10.57+17 100/4.58T. The data for τ from mixtures with 0.65≤[O2]/[C2H2]≤7.7 fit the expression log10([O2]τ) (mole sec/l)=−11.48+17 100/4.58T with a standard deviation of a factor of 1.4 in τ. In the first approximation the values of [O2]ti and [O2]τ are functions of temperature only. In lean mixtures, however, [O2]τ and especially [O2]ti are larger than the average. These results are explained in terms of a branching chain mechanism in which the reaction H+O2=OH+O plays the dominant role in controlling the rate. The CH4–O2 reaction was studied in the range of conditions 0.5≤[O2]/[CH4]≤4.5 and 1800≤T≤2700°K. The emission intensities are an order of magnitude smaller and more strongly temperature dependent, τ is a factor of 2 or 3 larger, and ti is a factor of 20 larger at the low temperatures and a factor of 4 larger at the high temperatures than for the C2H2–O2 reaction. The kinetic implications of these findings are discussed.

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