Dynamics of the O(3P) + C2H2 reaction from crossed molecular beam experiments with soft electron ionization detection

Abstract

The reaction between ground state oxygen atoms, O(³P), and the acetylene molecule, C₂H₂, has been investigated in crossed molecular beam experiments with mass-spectrometric detection and time-of-flight analysis at three different collision energies, E_c = 34.4, 41.1 and 54.6 kJ mol⁻¹. From product angular and velocity distribution measurements of the HCCO and CH₂ products in the laboratory frame, product angular and translational energy distributions in the center-of-mass frame were determined. Measurements on the CH₂ product were made possible by employing for product detection the recently implemented soft electron-ionization (EI) technique with low-energy, tunable electrons, which has permitted suppressing interference coming from the dissociative ionization of reactants, products and background gases. It has been found that the title reaction leads only to two competing channels: H + HCCO (ketenyl) and CO + CH₂ (triplet methylene). The branching ratio of cross sections between the two competing channels has been determined to be σ(HCCO)/[σ(HCCO) + σ(CH₂)] = 0.79 ± 0.05, independent of collision energy within the experimental uncertainty. This value is in line with that obtained in the most recent and accurate kinetics determination at room temperature as well as with that predicted from recent theoretical calculations based on statistical rate theory and weak-collision master equation analysis and on dynamics surface-hopping quasiclassical trajectory calculations on-the-fly on coupled triplet/singlet ab initio potential energy surfaces. The firm assessment of the branching ratio as a function of translational energy for this important reaction, besides its fundamental significance, is of considerable relevance for the implementation of theoretical models of hydrocarbon combustion.