Multiphoton-excited dynamics of the trans or cis structural isomer of 1,2-dibromoethylene

Photofragmentation dynamics of cis and trans isomers of 1,2-dibromoethylene (1,2-DBE) have been investigated by multiphoton excitation using a picosecond (ps) laser pulse. It has been found that the Br₂ ⁺ product ion preferentially originates from the cis isomer rather than from trans. The Boltzmann-type isotropic low kinetic energy components of the Br⁺ and Br₂ ⁺ product state distributions seem to be most likely from the unimolecular reactions of the vibrationally hot cationic ground state generated by the three-photon absorption at the photon energy below ∼38 000 cm^-1. The highly anisotropic kinetic energy components of Br⁺ and Br₂ ⁺ start to appear at the photon energy above ∼38 000 cm^-1, where the D_n (n ≥ 1) - D₀ transition is facilitated within the same ps laser pulse as the parent molecule is efficiently ionized by the two-photon absorption. The transition dipole moment of the D₄ - D₀ transition of the strongest oscillator strength has been theoretically predicted to be parallel to the C-Br bond or C=C bond axis for the trans or cis isomer, respectively. The fast anisotropic with the (β ∼ +2) component in the Br⁺ product distribution is thus likely from the trans isomer, whereas that of Br₂ ⁺ (β ∼ -0.5) should be the consequence of the photodissociation of the cis isomer. The isomer-specific reactivity found here in the picosecond multiphoton excitation of 1,2-DBE provides a nice platform for the better understanding of the structure-reactivity relationship under the harsh condition of the strong or ultrashort optical field.