| Summary: | 碩士 === 國立臺灣大學 === 化學研究所 === 105 === 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.
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