苯酚的光分解：光解碎片動能分布中慢速產物的來源

Abstract

因為苯酚是多重位能曲面分解的重要模型分子，其光分解在過去數十年來被許多實驗與理論計算研究過。苯酚在紫外光區域光分解的主要碎片為OH斷鍵產生的氫原子與苯氧基。先前的實驗在測量光解碎片動能分布時，發現有兩個不同的產物，他們分別被稱為快速產物與慢速產物。快速產物被認為是苯酚的激發態分解成苯氧基的基態。而慢速產物則有以下兩種可能：(1) 從苯酚的基態分解而來，(2) 從苯酚的激發態分解成苯氧基的激發態。過去並沒有實驗測量這兩種分解途徑的分支比。 在本實驗中，我們藉由改良傳統的光解碎片動能儀完成一新型態的時間解析實驗並得到時間解析的移動動能分布。藉此我們可以很清楚地分出三種不同的分解途徑。分解途徑一在移動動能分布中會在12000cm-1有一峰值，並且其生命週期為小於10奈秒，因此分解途徑一為苯酚的激發態分解成苯氧基的基態；分解途徑二在移動動能分布中會在2000cm-1有一峰值，同樣的其生命週期為小於10奈秒，因此分解途徑二為苯酚的激發態分解成苯氧基的激發態；分解途徑三在移動動能分布中主要落於小於3000cm-1的區域，而其生命週期為大於100奈秒，因此分解途徑三為苯酚的基態分解。根據本實驗的結果，我們可以得到苯酚在基態分解及苯酚在激發態分解成苯氧基的基態、第一激發態、第二激發態的分支比。在193奈米分解時分別為0.05、0.53、0.24和0.17；在213奈米分解時分別為0.07、0.60、0.32和0。我們得到的這些分支比數值可以做為檢驗理論計算結果的重要依據。而我們所開發出的新方法也有助於研究其他有多重位能曲面分解的分子的光分解。

Photodissociation of phenol has been investigated by both experimental and theoretical methods for the past few decades since it is an important model molecular of multistate dissociation. The major photofragments in UV region are Hydrogen atom plus phenoxyl radical produced through OH bond fission. Previous experiments showed that there are two components, namely fast and slow, in the photofragment translational energy distributions. The fast component was assigned as dissociation in the electronic excited state forming the ground state phenoxyl radical. As for the slow component, it can be assigned as (1) dissociation in the electronic excited state forming the excited state phenoxyl radical, or (2) internal conversion followed by dissociation in the electronic ground state. There was no experimental measurement of branching ratio for these two channels before. In this work, we have performed a new type of time-resolved experiment using modified conventional photofragment translational spectroscopy to get the time-resolved spectra of photofragment translational energy. The results show clear characteristic of three different dissociation channels. The first channel producing a component centered at ~12000 cm-1 in translational energy distribution has a lifetime < 10 ns, and is assigned as dissociation in the excited state forming the ground state phenoxyl radical. The second channel generates a component centered at ~2000 cm-1 in translational energy distribution with a lifetime < 10 ns, and is assigned as dissociation in the excited state forming the excited state phenoxyl radical. The third channel producing a component mainly below 3000 cm-1 in translational energy distribution has a lifetime > 100 ns, and is assigned as dissociation in the ground state forming the excited state phenoxyl radical. Finally, we get the branching ratio of ground state dissociation channel and excited state dissociation channel forming phenoxyl radical X,A,B state for the photodissociation of phenol at 193nm as 0.05, 0.53, 0.24, 0.17, and at 213 nm as 0.07, 0.60, 0.32, 0. These branching ratios are useful for justifying the results of theoretical calculations. Furthermore, this technique is useful for the investigation of photodissocition of other molecules which would also dissociate on the multi potential energy surface.