光分解之非絕熱機制與斷鍵選擇性探討

徐明義

doi:10.6342/NTU.2012.00321

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光分解之非絕熱機制與斷鍵選擇性探討

Nonadiabatic Effect on the Branching Ratio between C-Br and C-Cl Bond Fission in Photodissociations of Bromoacetyl Chloride, 2- and 3-Bromopropionyl Chlorides

徐明義 , Masters　　Advisor：林金全　　

繁體中文 DOI： 10.6342/NTU.2012.00321

溴乙醯氯；光分解；共振增強多光子游離；速度離子影像；分子動態學； bromoacetyl chloride ； photodissociation ； REMPI ； veloticy map ion image ； molecular dynamics

Abstract │ Reference (92) │ Altmetrics

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We conduct the photodissociation experiments of bromoacetyl chloride and two isomers of bromopropionyl chlorides by using (2+1) resonance-enhanced multiphoton ionization (REMPI) technique combined with velocity map ion imaging. Our goal is to study Norrish type I a-bond cleavage at 234~235 nm via a 1[n(O) --> p*(C=O)] transition. The parent molecules are photoexcited to lowest electronic excited state S1 causing the C-Br or C-Cl bond breaking and we can measure the bond fission branching by the ion intensity. The bromoacetyl chloride, 2- and 3-bromopropionyl chlorides yielding the branching ratios of C-Br/C-Cl bond fission is 0.47, 0.35, and 0.047 respectively. Results from this study qualitatively agree with Bulter’s previous studies at 193 nm. Their branching ratios of C-Br/C-Cl are 0.4 and <0.05 for bromoacetyl chloride and 3-bromopropionyl chloride, where they utilized similar method but the statistical predictions is 30 and 2 respectively. They conclude that its results are not controlled by the relative exit barriers of adiabatic PESs, but by the probatility of nonadiabaticity. As suggested previously, the C-Cl bond fission is anticipated to follow an adiabatic reaction curve, for a strong diabatic coupling between the n(O)π*(C=O) and np(Cl)σ*(C-Cl) bands. In contrast, for the C-Br bond fission which is subject to much weaker coupling strength between n(O)π*(C=O) and np(Br)σ*(C-Br), a diabatic pathway is preferred for bromoacetyl chloride and 2-bromopropionyl chloride leading to the excited state products that are energetically accessible. For 3-bromopropionyl chloride, the available energy cannot reach the excited state products such that the C-Br bond fission has to proceed via the adiabatic pathway with severe nonadiabatic suppression. The proposed mechanisms are supported by observation of a small fraction of Br translational energy disposal and the energy calculations of excited state products. The spin-orbit state-specific translational energy and angular distributions of halogen atoms resulting from the competitive bond fission are also obtained. Aside from the atomic fragments, the CO fragments are detected and believed to result from a secondary decomposition of the moieties with enough internal energy deposition.

Reference ( 92 ) 〈TOP〉

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