光分解之非絕熱機制與斷鍵選擇性探討

Abstract

在本論文中利用(2+1)共振增強多光子游離（resonance-enhanced multiphoton ionization，REMPI）結合離子速度影像（velocity ion imagimg），來研究一系列的溴化醯氯分子在234~235nm波長下的Norrish type I光分解反應。反應物吸光後經由1[n（O）π*（C=O）]躍遷至S1激發態，斷鍵生成Cl或Br原子。我們所測得的溴乙醯氯、2-溴丙醯氯與3-溴丙醯氯的C-Br/C-Cl斷鍵分支比（branching ratio）依序是0.47、0.35與0.047，參考Butler所發表的文獻中，溴乙醯氯與3-溴丙醯氯的C-Br/C-Cl斷鍵分支比分別為0.4與<0.05，顯示我們的實驗值符合其他文獻。但根據統計理論計算的預測，溴乙醯氯與2-溴丙醯氯的C-Br/C-Cl斷鍵分支比應約為30與2，因此只考慮分子在絕熱位能面（adiabatic PES）下光解的位能障礙是不夠的，還必須考慮非絕熱躍遷（nonadiabatic transition）對斷鍵選擇性造成的影響。 原子碎片的分支比主要受兩點影響：（1）光解路徑（2）分子內Br與CO發色團的距離。正如先前文獻所解釋的，因為n(O)π*(C=O)與np(Cl)σ*(C-Cl)位能面的非絕熱耦合能（diabatic coupling）很大，使得C-Cl斷鍵來自於分子躍遷至S1激發態後沿著絕熱路徑分解；另一方面n(O)π*(C=O)與np(Br)σ*(C-Br)的非絕熱耦合能很小，對於溴乙醯氯和2-溴丙醯氯而言，C-Br斷鍵會傾向於S1激發態經由非絕熱路徑產生激發態產物，而非先前文獻所推測的此通道被抑制了，然而3-溴丙醯氯光分解後的可用能量不足以生成激發態產物，造成此通道被抑制了。以上所推測的光解路徑均有原子碎片的動能分佈圖與激發態產物能量計算來佐證。 除此之外本論文還測得二次分解所產生的CO碎片，以及基態與激發態原子碎片的分支比，而此分支比必須仰賴含有自旋軌道耦合作用力（spin-orbit coupling）的理論計算才可解釋。

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.