Reactive scattering of oxygen atoms with unsaturated hydrocarbons: Primary products, branching ratios and role of intersystem crossing

We have investigated the combustion relevant reaction dynamics of ground state oxygen atoms with small unsaturated hydrocarbons (acetylene, ethylene and allene) using the crossed molecular beams method with “universal” mass spectrometric detection and time-of-flight analysis. By exploiting “soft” electron-ionization detection we have identified and characterized the dynamics of the energetically allowed channels in each of the above reactions. In particular, we have explored how the dynamics, branching ratios and extent of inter-system-crossing (ISC) from triplet to singlet potential energy surfaces (PESs) vary with collision energy. The role of ISC is found to increase strongly with increasing complexity of the hydrocarbon. The experimental results have been compared with theoretical predictions (when available), both statistical calculations and quasiclassical-trajectory surface-hopping computations on coupled ab initio triplet and singlet PESs, carried out in other laboratories.