Negative and secondary ion formation in low-pressure flames

Abstract

Mass spectrometry and Langmuir probes have been used in a study of negative- and secondary positive-ion formation in low-pressure flames. Individual positive- and negative-ion profiles have been examined as a function of pressure and flame composition. Two mechanisms for secondary positive ion formation are proposed: The first consists of a series of charge-exchange reactions involving the primary ion (e.g., CHO + ) and stable neutral combustion intermediates: CHO + +CH 2 O→CH 2 OH + +CO CHO + +CH 3 OH→CH 3 OH 2 + +CO CHO + +CH 3 CHO→CH 3 CHOH + +CO CHO + +C 2 H 2 O→C 2 H 3 O + +CO. The plausibility of these reactions has been demonstrated by addition of CH 2 O, CH 3 OH, and CH 3 CHO to atomic diffusion flames. The second mechanism is one in which a primary ion undergoes a chain of oxidative degradation reactions which includes the observed ions as intermediates. Addition of N 2 and CO 2 to flames, together with the observed effects of varying flame composition, has led to the conclusion that, in addition to CHO + , an important primary ion is C 3 H 3 + . The proposed process responsible for the formation of C 3 H 3 + is: CH * +C 2 H 2 →C 3 H 3 + + e . Langmuir probe studies of a C 3 H 8 -air flame at 40 Torr have confirmed the importance of diffusional loss mechanisms in low-pressure flames and have led to the conclusion that the recombination coefficient α is temperature-independent. The measured values for the recombination coefficient and ambipolar diffusion coefficient are: α =2.4×10 −7 cm 3 /sec; D a =60 cm 2 /sec (referred to 1 Torr, 298°K). Taken together with values in the literature we conclude that α =2.4±0.4×10 −7 cm 3 /sec independent of temperature and pressure. Mass-spectrometric negative-ion profiles demonstrate, for the first time, the presence of significant quantities of negative ions in flames at 1 to 3 Torr. Dissociative attachment processes are the most probable formation route, and arguments are presented for C 2 − as the first negative ion formed via e +C 2 H 2 O * →C 2 − +H 2 O.