Chemi-ionization in oxyacetylene flames

Abstract

The identity of the first ion formed in hydrocarbon flames has created much discussion. Two species have been favoured^1–7, produced in the chemi-ionization processes: CH + O → CHO⁺ + e⁻ (1) CH^* + C₂H₂ → C₃H₃⁺ + e⁻ (2) Reaction (2) is usually taken to involve electronically excited CH, in which case reaction (2), like reaction (1) with ground state CH, is close to thermoneutral. Experimental evidence has been claimed for both. For example, Peeters et al.³ prefer reaction (1), while Calcote et al.⁴ have suggested that reaction (2) is necessary to explain the abundance of ions in fuel-rich flames. Interestingly, evidence for CHO⁺ has been obtained^1,3,5 from either non-acetylene flames or those with low C/O ratios, while evidence for C₃H₃⁺ appears^2,4,6,7 in fuel-rich acetylene flames, where [C₂H₂] is high and [O] is low. We describe here a mass spectrometric study of fuel-rich acetylene flames, and attempt to clarify the picture. Evidence is presented which suggests that both reactions operate simultaneously in oxyacetylene flames.