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 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 similar to 38 000 cm(-1). The highly anisotropic kinetic energy components of Br+ and Br-2(+) start to appear at the photon energy above similar to 38 000 cm(-1), where the D-n (n >= 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 similar to +2) component in the Br+ product distribution is thus likely from the trans isomer, whereas that of Br-2(+) (beta similar to -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.

Automated summary

The photofragmentation dynamics of the cis and trans isomers of 1,2-dibromoethylene were studied using multiphoton excitation, revealing isomer-specific reactivity. The Br-2(+) product ion from the cis isomer preferentially originates from a specific photon energy for unimolecular reactions, while anisotropic kinetic energy components of Br+ and Br-2(+) from the trans isomer appear at higher photon energies. The transition dipole moment of the strongest oscillator strength indicates the orientation of the transition for the trans and cis isomers, leading to fast anisotropic components in the product distributions.