Ion formation in the photochemically initiated combustion of C 2H 2-O 2-NO 2 mixtures : Part II. Ion current and emission spectra

Abstract

Mixtures of C 2 H 2 -O 2 -NO 2 in a silica tube were ignited by a xenon flash lamp adjacent to the tube and the time change of the C 2 , CH, and OH emission spectra from the mixtures was studied, together with the time change of the ion current through them. The emission and absorption spectra and the ion current showed nothing significantly different from those of ordinary C 2 H 2 -O 2 flames, and it was concluded that the main combustion reactions and the mechanism of ion production, that is, CH+O→CHO + + e , are essentially common to both cases. The ion current never failed to start more than 50 μsec after the first radical appeared. The first radical to appear was CH * when the combustion tube was long, and C 2 * when it was short, the interval between them being roughly 30 μsec. The relation of C 2 * and CH * is not such that one of them originates from the other. They were considered to come from a common species—probably polymerized C 2 H. The C 2 thus produced does not seem to be directly linked with ion production, although it is responsible for ion formation through CH by the reaction: C 2 +OH→CH+CO. Radical emission and absorption, as well as ion production, were greatly dependent upon the mixture ratio of the gases. This dependence is discussed and it is concluded that CH * (2) is more closely related to CHO + formation than is CH( 2 Π). Based on our experimental results, a system of reactions for producing CHO + has also been proposed.