A combined ab initio and photoionization mass spectrometric study of polyynes in fuel-rich flames

Polyynic structures in fuel-rich low-pressure flames are observed using VUV photoionization molecular-beam mass spectrometry. High-level ab initio calculations of ionization energies for C_2nH₂ (n = 1–5) and partially hydrogenated C_nH₄ (n = 7–8) polyynes are compared with photoionization efficiency measurements in flames fuelled by allene, propyne, and cyclopentene. C_2nH₂ (n = 1–5) intermediates are unambiguously identified, while HCC–CC–CHCCH₂, HCC–CC–CC–CHCH₂ (vinyltriacetylene) and HCC–CC–CHCH–CCH are likely to contribute to the C₇H₄ and C₈H₄ signals. Mole fraction profiles as a function of distance from the burner are presented. C₇H₄ and C₈H₄ isomers are likely to be formed by reactions of C₂H and C₄H radicals but other plausible formation pathways are also discussed. Heats of formation and ionization energies of several combustion intermediates have been determined for the first time.