Abstract
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(Et) and the anisotropy parameters β 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 β 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 (π,π*) state.
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