Effects of Methyl\nSubstitution on the Ultrafast Internal\nConversion of Benzene

The effects of methyl substitution on the ultrafast internal\nconversion\nfrom the S<sub>2</sub>(<sup>1</sup>B<sub>1u</sub>, ππ*)\nstate to the S<sub>0</sub> state of benzene were studied using ultrafast\nextreme-ultraviolet photoelectron spectroscopy and electronic structure\ncalculations. The quantum yield of the internal conversion to the\nS<sub>0</sub> state reached ∼0.69 in benzene, while lower values\nof 0.35 and 0.12 were obtained for toluene and <i>o</i>-xylene,\nrespectively. These results indicate that methyl substitution makes\nthe conical intersections less accessible to the nuclear wave packet.