Experimental probing of conical intersection dynamics in the photodissociation of thioanisole.

Chemical reactions that occur in the ground electronic state are described well by invoking the Born-Oppenheimer approximation, which allows their development to be rationalized by nuclear rearrangements that smoothly traverse an adiabatic potential energy surface. The situation is different, however, for reactions in electronically excited states, where non-adiabatic transitions occur between adiabatic surfaces. The conical intersection, in which two adiabatic surfaces touch, is accepted widely as the dynamic funnel for efficient non-adiabatic transitions, but its direct experimental probing is rare. Here, we investigate the photodissociation of thioanisole and observe a striking dependence of the relative yields of two reaction channels on the photoexcitation energy as indicated by a dynamic resonance in the product branching ratio. This results from the interference of two different adiabatic states that are in close proximity in the region of a conical intersection. The location of the observed resonance on the multidimensional potential energy surface thus reveals the nuclear configuration of the conical intersection and its dynamic role in the non-adiabatic transition.