The positive and negative ions in oxy-acetylene flames

The positive and negative ions in atmospheric pressure flames of C₂H₂ + O₂ + N₂ have been examined mass spectrometrically with the much improved sampling systems now available. There is clear evidence that some (but not all) ionic reactions are fast enough to proceed in the time of around 1 μs taken by the sample to enter the instrument, leading to a falsification of the observed spectra. Even so, it is possible to conclude that there is strong evidence for C₃H₃⁺ being a primary ion in C₂H₂ flames, in addition to CHO⁺. The first negative ion to be formed in the reaction zone is O₂^- as a result of electron attachment to O₂, but there is a possibility that C₂H^- is also formed from electrons by dissociative attachment to C₂H₂. Some of the subsequent reactions of both positive and negative ions are discussed. The mass spectra for positive and negative ions reveal the presence of large neutral hydrocarbon species, such as C_n, C_nH₂ and C_nH_y with molecular weights of at least 100; these are presumably associated with carbon formation. In addition, positive and negative nitrogenous ions reflect the formation of NO, CN, NCO, NH₃, and in particular, the production of NO by the 'prompt' mechanism. The major positive ion in the burnt gases is NO⁺, and because recombination in NO⁺ + e^- → N + O becomes the principal reaction for charged species, it has proved possible to measure its rate coefficient as 4.2 ± 2.5 × 10^-7 ions^-1 ml s^-1 at 2600 K.