Effects of Methyl Substitution on the Ultrafast Internal Conversion of Benzene.

The effects of methyl substitution on the ultrafast internal conversion from the S2(1B1u, 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.