Photodissociation of CHCCH2X (X=Br and Cl) by translational spectroscopy

The photodissociation of CH=CCH2X (X = Br and Cl) at 248 and 193 nm was studied using an angle-resolved beam apparatus. Both the translational energy distribution P(E-t) and the anisotropy parameters beta of the photoproducts were derived from the time-of-flight spectra. Excitation at 248 nm, CH=CCH2Br underwent exclusively a direct C-Br bond fission with anisotropic recoiling fragments. By increasing the photon energy to 193 nm, three competing primary dissociation channels were observed: (1) Br + C3H3 (fast, 26%), (2) Br + C3H3 (slow, 44%), and (3) HBr + C3H2 (30%), with average translational energies of 34, 12, and 9 kcal/mol, respectively. From the measured beta values, all reactions were shown to proceed extremely fast within a fraction of rotational period. On the other hand-excitation at 193 nm, CH=CCH2Cl underwent a predominant C-Cl bond fission with a minor channel producing the HCl + C3H2 fragments. The results are discussed in terms of either a direct or an indirect dissociation process arising from the transition to the (pi, pi*) state. (C) 1998 American Institute of Physics.