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

Abstract

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 Br-2(+) product ion preferentially originates from the cis isomer rather than from trans. The Boltzmann-type isotropic low kinetic energy components of the Br+ and Br-2(+) 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 & SIM;38 000 cm(-1). The highly anisotropic kinetic energy components of Br+ and Br-2(+) start to appear at the photon energy above & SIM;38 000 cm(-1), where the D-n (n & GE; 1) - D-0 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 D-4 - D-0 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 (beta & SIM; +2) component in the Br+ product distribution is thus likely from the trans isomer, whereas that of Br-2(+) (beta & SIM; -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.</p>