The positive ion chemistry in the post-combustion zone of sooting premixed acetylene low pressure flat flames

The results of flame-ion mass spectrometry are used to study the positive ion chemistry in the post-combustion zone of sooting low-pressure acetylene flat flames for which we have measured the neutral composition, or whose neutral concentration profiles are available from previous workers. The post-combustion zone of rich acetylene flat flames is characterized by large amounts of unburnt fuel from which polyacetylenes and polycyclic aromatic hydrocarbons (PCAH) are formed by radical reactions. In this zone, we have detected no oxygenated ions, only C n H m + hydrocarbon ions, whose formulas were determined by burning rich C 2 D 2 flames. We have confirmed experimentally that the hydrocarbon ions are genuine flame ions, and are not formed in the sampling process. C 3 H 3 + dominates in the flame ion mass spectrum, representing about 80% of the total ion signal, and its concentration decreases slowly downstream of the flame front. The concentration profiles of the other C n H m + ions exhibit the same decay as for C 3 H 3 + . From the analysis of the neutral and ionic species profiles, we were able to establish that some ion-molecule reactions involving C 3 H 3 + and neutral species are at equilibrium at the flame temperature. The C 2n+1 H 3 + ions (with n =2,3 and 4) derive from balanced reaction with C 3 H 3 + and hydrogen, acetylene and polyacetylenes from which the thermochemical properties of the C 2n+1 H 3 + ions, which are not available in the literature, have been obtained. The larger hydrocarbon ions have a probable aromatic structure and are believed to be very stable. It has been shown that the formation of some of them can be explained by balanced ion-molecule reactions involving C 3 H 3 + and aromatic species, from which the thermochemical properties of aromatic ions were obtained.