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 b eam. Using momentum conservation, we show that the primary product is singl et vinylidene [H2CC((X) over tilde(2)A')], or singlet acetylene at energies where interconversion between the H2CC and HCCH geometries is facile. In a ddition, a minor contribution is seen which is assigned to triplet acetylen e [C2H2((a) over tilde(3)B(2))]. We argue that out-of-plane motion of the t hird H atom is necessary to bring the excited states, of A" symmetry, to an A' symmetry leading to products, and the observed tranlsational energy dis tribution may show evidence of the barrier to this process. The heat of for mation of vinylidene is derived to be 100.3 +/- 4.0 kcal/mol, in agreement with literature values. From the translational energy release, we derive th e T-0 for triplet acetylene C2H2((a) over tilde(3)B(2)) to be 28900 cm(-1), which does not agree well with recent ab initio calculations. Possible rea sons for the disagreement are discussed. (C) 1999 American Institute of Phy sics. [S0021-9606(99)00709-6].