Photodissociation of 1,2-dibromoethylene at 248 nm:: Br2 molecular elimination probed by cavity ring-down absorption spectroscopy

The Br-2 elimination channel is probed for 1,2-C2H2Br2 in the B-3 Pi(+)(ou)-X-1 Sigma(+)(g) transition upon irradiation at 248 nm by using cavity ring-down absorption spectroscopy (CRDS). The nascent vibrational population ratio of Br-2(v=1)/Br-2(v=0) is obtained to be 0.7 +/- 0.2, thus indicating that the Br-2 fragment is produced in hot vibrational states. The obtained Br-2 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 Br-2. According to ab initio potential energy calculations, the pathways that lead to Br-2 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 Br-2 elimination reaction is determined to be 0.12 +/- 0.1, being substantially contributed by the ground-state Br-2 product. The discrepancy of this value from that (of 0.2) obtained by PTS may rise from the lock of measurements in probing the triplet-state Br-2 product.