Real-Time Observation of Nonadiabatic Bifurcation Dynamics at a Conical Intersection.

Looking into temporal dynamics of the reactive flux that is precisely located at the well-characterized conical intersection has been one of chemists' longstanding goals. We report here real-time nonadiabatic bifurcation dynamics in the S-CH3 bond predissociation of thioanisole (C6H5SCH3) in the first electronically excited state (S1). It is found that two distinct adiabatic and nonadiabatic reaction pathways are activated simultaneously only when the vibronic state near the first conical intersection is optically accessed. Our time-resolved measurement of the product state distribution could separate two different dynamic channels unambiguously, unraveling the detailed dynamic mechanism of the nonadiabatic reaction taking place in the vicinity of the conical intersection. The nonadiabatic channel, where the reactive flux funnels through two consecutive conical intersections along the reaction coordinate, is found to be significantly faster than the adiabatic channel along the minimum energy reaction pathway. The kinetic energy release ratio and the nonadiabatic transition probability are found to be much higher for the nonadiabatic channel than those of the adiabatic channel, giving insights into the bifurcation dynamics occurring at the conical intersection.