The effects of methyl substitution on the ultrafast internal conversion from the S₂(¹B_1u, ππ*) state to the S₀ state of benzene were studied using ultrafast extreme-ultraviolet photoelectron spectroscopy and electronic structure calculations. The quantum yield of the internal conversion to the S₀ state reached ∼0.69 in benzene, while lower values of 0.35 and 0.12 were obtained for toluene and o-xylene, respectively. These results indicate that methyl substitution makes the conical intersections less accessible to the nuclear wave packet.

中文翻译：

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甲基取代对苯超快内转化的影响


采用超快极紫外光电子能谱和电子结构计算研究了甲基取代对苯从 S₂（¹B_1u， ππ*） 状态到 S₀ 状态的超快内部转化的影响。在苯中，内部转化为 S₀ 态的量子产率达到 ∼0.69，而甲苯和邻二甲苯分别获得了 0.35 和 0.12 的较低值。这些结果表明，甲基取代使核波包更难接近圆锥形交点。