The photodissociation of the vinyl radical (C2H3) at 243 nm studied by velocity map imaging

The technique of velocity map imaging (VELMI) has been applied to study the photodissociation of the vinyl radical (C2H3) at 243.2 nm in a molecular beam. Using momentum conservation, we show that the primary product is singlet vinylidene [H2CC(X̃ 2A′)], or singlet acetylene at energies where interconversion between the H2CC and HCCH geometries is facile. In addition, a minor contribution is seen which is assigned to triplet acetylene [C2H2(ã 3B2)]. We argue that out-of-plane motion of the third H atom is necessary to bring the excited states, of A″ symmetry, to an A′ symmetry leading to products, and the observed tranlsational energy distribution may show evidence of the barrier to this process. The heat of formation of vinylidene is derived to be 100.3±4.0 kcal/mol, in agreement with literature values. From the translational energy release, we derive the T0 for triplet acetylene C2H2(ã 3B2) to be 28 900 cm−1, which does not agree well with recent ab initio calculations. Possible reasons for the disagreement are discussed.