Photodissociation of 1,2-dibromoethylene at 248 nm: Br2 molecular elimination probed by cavity ring-down absorption spectroscopy
Resource
ChemPhysChem 9 (8): 1137-1145
Journal
ChemPhysChem
Journal Volume
9
Journal Issue
8
Pages
1137-1145
Date Issued
2008
Author(s)
Chang, Y.-P.
Lee, P.-C.
KING-CHUEN LIN
Huang, C.H.
Sun, B.J.
Chang, A.H.H.
Abstract
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.
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journal article
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