Shock-Tube Study of the Acetylene–Oxygen Reaction. II. Chemi-ionization during the Induction Period

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Abstract
The exponential growth of CH*(A 2Δ → X 2Π) emission and chemi-ion production were simultaneously monitored behind reflected shock waves during the induction period of the C2H2+O2 reaction. Experiments were performed in the temperature range 1479–2282°K using three C2H2+O2+Ar gas mixtures whose C2H2:O2:Ar proportions were 5:5:990, 5:10:985, and 5:30:965. End-on observation was used to monitor CH* emission, and a new saturation-current technique was developed and used to monitor the rate of chemi-ion production. In each experiment the exponential growth constant for ion production was twice that for CH* emission. The results establish that chemi-ionization during the C2H2+O2 induction period is second order in reaction intermediates, which rules out the reaction CH*+C2H2→C3H3++e and is consistent with reaction CH+O→CHO++e as the major chemi-ionization reaction during this early portion of the reaction. An ion–molecule reaction mechanism together with the data obtained in this study is used to demonstrate that the predominance of C3H3+ during the induction period can be explained by such a mechanism.