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 Cn, CnH2 and CnHy 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.
Bibliographical noteFunding Information:
D. B. K. gratefully acknowledges financial support from the U. S. Churchill Foundation, Churchill College and the U.S. National Science Foundation.