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题名: Molecular Elimination of Br2 in 248 nm Photolysis of Bromoform Probed by Using Cavity Ring-down Absorption Spectroscopy
作者: Huang, H. Y.;Chuang, W. T.;Sharma, R. C.;Hsu, C. Y.;Lin, K. C.;Hu, Ching-Han
贡献者: 化學系
日期: 2004-09
上传时间: 2012-05-03T06:17:46Z
出版者: American Institute of Physics
摘要: By using cavity ring-down spectroscopy technique, we have observed the channel leading to Br2 molecular elimination following photodissociation of bromoform at 248 nm. A tunable laser beam, which is crossed perpendicular to the photolysis laser beam in a ring-down cell, is used to probe the Br2 fragment in the B 3Πou+−X 1Σg+ transition using the range 515–524 nm. The ring-down time lasts 500 ns, so the rotational population of the Br2 fragment may not be nascent nature, but its vibrational population should be. The vibrational population ratio of Br2(v = 1)/Br2(v = 0) = 0.8±0.2 implies that the fragmented Br2 is vibrationally hot. The quantum yield of the molecular elimination reaction is 0.23±0.05, consistent with the values of 0.26 and 0.16 reported in 234 and 267 nm photolysis of bromoform, respectively, using velocity ion imaging. A plausible photodissociation pathway is proposed, based upon this work and ab initio calculations. The  1A2,  1E, and  1A1 singlet states of bromoform are probably excited at 248 nm. These excited states may couple to the high vibrational levels of the ground state  1A1 via internal conversion. This vibrationally excited bromoform readily surpasses a reaction barrier 389.6 kJ/mol prior to decomposition. The transition state structure tends to correlate with vibrationally hot Br2. Dissociation after internal conversion of the excited states to vibrationally excited ground state should result in a large fraction of the available energy to be partitioned in vibrational states of the fragments. The observed vibrationally hot Br2 fragment seems to favor the dissociation pathway from high vibrational levels of the ground state. Nevertheless, the other reaction channel leading to a direct impulsive dissociation from the excited states cannot be excluded.
關聯: J. Chem. Phys., 121(11): 5253-5260
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