Molecular beam studies of combustion reactions of oxygen atoms with unsaturated hydrocarbons: Primary products, branching ratios and central role of intersystem crossing

The reactions of ground state oxygen atoms with unsaturated hydrocarbons are of central importance in combustion. We have investigated the O-atom reactions with acetylene, ethylene, allene and methylacetylene using the crossed beams method with universal mass spectrometric detection empowered by soft electron-ionization to characterize the dynamics of the energetically allowed channels and explore how primary products, branching ratios and extent of Inter-System-Crossing (ISC) vary with molecular complexity, electronic structure and collision energy. ISC from triplet to singlet potential energy surfaces (PESs) is found to play a central role in most of these reactions. Experimental results are compared with those of theoretical predictions by statistical calculations (available for all systems) and those (in coll. with Bowman’s group) of quasiclassical-trajectory surface-hopping computations on coupled ab initio triplet and singlet PESs (for the benchmark multichannel nonadiabatic reaction O+ethylene). The relevance of the results in the theoretical modelling of combustion is noted.