Br(2) molecular elimination is probed in the photodissociation of 1,1- and 1,2-C(2)H(4)Br(2) isomeric forms at 248 nm by using cavity ring-down absorption spectroscopy. Their photodissociation processes differ markedly from each other. The quantum yield of the Br(2) fragment in 1,2-dibromoethane is 0.36+/-0.18, in contrast to a value of 0.05+/-0.03 in 1,1-dibromoethane. The vibrational population ratios of Br(2)(v=1)/Br(2)(v=0) are 0.8+/-0.1 and 0.5+/-0.2 for 1,2- and 1,1-dibromoethanes, respectively. The Br(2) yield densities are found to increase by a factor of 35% and 190% for 1,2- and 1,1-dibromoethanes within the same temperature increment. In the ab initio potential energy calculations, the transition state (TS) along the adiabatic ground state surface may correlate to the Br(2) products. The TS energy for 1,2-dibromoethane is well below the excitation energy at 483 kJ/mol, whereas that for 1,1-dibromoethane is slightly above. Such a small TS energy barrier impedes the photodissociation of the ground state 1,1-dibromoethane such that the production yield of Br(2) may become relatively low, but rise rapidly with the temperature. The TS structure shows a larger bond distance of Br-Br in 1,2-dibromoethane than that in 1,1-dibromoethane. That explains why the former isomer may result in hotter vibrational population of the Br(2) fragments.

The Br2 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 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 0.120.1, 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.

In the photodissociation of 1,1-C(2)H(2)Br(2) at 248 nm, the Br(2) elimination channel is probed by using cavity ring-down absorption spectroscopy (CRDS). In terms of spectral simulation, the vibrational population ratio of Br(2)(v = 1)/Br(2)(v = 0) is found to be 0.55+/-0.05, which indicates that the Br(2) fragment is vibrationally hot. The rotational population is thermally equilibrated with a Boltzmann temperature of 349+/-38 K. According to ab initio potential energy calculations, the obtained fragments are anticipated to result primarily from photodissociation of the ground electronic state that undergoes 1) H migration followed by three-center elimination, and 2) isomerization forming either trans- or cis-1,2-C(2)H(2)Br(2) from which Br(2) is eliminated. RRKM calculations predict that the Br(2) dissociation rates through the ground singlet state prevail over those through the triplet state. Measurements of temperature and Ar pressure dependence are examined to support the proposed pathway via internal conversion. The quantum yield for the Br(2) elimination reaction is determined to be 0.07+/-0.04. This result is smaller than that obtained in 1,2-C(2)H(2)Br(2), probably because the dissociation rates are slowed in the isomerization stage.

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