Photodissociation of 1,2‐Dibromoethylene at 248 nm: Br2 Molecular Elimination Probed by Cavity Ring‐Down Absorption Spectroscopy

The Br₂ elimination channel is probed for 1,2‐C₂H₂Br₂ in the B –X transition upon irradiation at 248 nm by using cavity ring‐down absorption spectroscopy (CRDS). The nascent vibrational population ratio of Br₂(v=1)/Br₂(v=0) is obtained to be 0.7±0.2, thus indicating that the Br₂ fragment is produced in hot vibrational states. The obtained Br₂ products are anticipated to result primarily from photodissociation of the ground‐state cis isomer via four‐center elimination or from cis/trans isomers via three‐center elimination, each mechanism involving a transition state that has a Br? Br distance much larger than that of ground‐state Br₂. According to ab initio potential energy calculations, the pathways that lead to Br₂ elimination may proceed either through the electronic ground state by internal conversion or through the triplet state by intersystem crossing. Temperature‐dependence measurements are examined, thereby supporting the pathway that involves internal conversion—which was excluded previously by using product translational spectroscopy (PTS). The quantum yield for the Br₂ elimination reaction is determined to be 0.12±0.1, being substantially contributed by the ground‐state Br₂ product. The discrepancy of this value from that (of 0.2) obtained by PTS may rise from the lack of measurements in probing the triplet‐state Br₂ product.