Guided-ion beam cross section and product ion time-of-flight (TOF) measurements are presented for the O+(4S)+C2H2 reaction over the center-of-mass collision energy range of 0.05–18 eV. Despite a large number of exothermic channels, the total reaction cross section at low energies is more than two orders of magnitude smaller than the capture cross section. A common energy onset for charge-transfer, CH+, and COH+/HCO+ products is observed at 1.70±0.10 eV, above which the total cross section for these channels rapidly rises with energy, eventually exceeding 5 Å2. Above 4 eV, the C2H+ and CO+ products also become significant, and weaker channels producing C+, C2+, and CH2O+ are also identified. The C2H+ fragment is interpreted as being primarily a dissociative charge transfer (CT) channel at collision energies above ∼3.79 eV, while the threshold for forming CO+ can be associated with a CO++H+CH channel. The TOF measurements demonstrate that efficient C2H2+ Ã state formation occurs above ∼2.7 eV. The onset at 1.70±0.10 eV is attributed to a transition state associated with an excited quartet hypersurface.

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