Herzberg Type-I or Type-II Predissociation Dynamics at the Conical Intersection: Bifurcation into Adiabatic or Nonadiabatic Pathway.

Bifurcation dynamics into Herzberg type-I and type-II predissociation pathways at the conical intersection have been characterized in terms of their distinct reaction rates and energy-disposal dynamics from the picosecond time-resolved product state distributions measured at multiple S1/S2 vibronic states of thioanisole (C6H5SCH3). Electronic predissociation (type-I) occurs on a faster time scale and leads to the larger translational energies being released to the fragments (C6H5S• + •CH3) compared to the vibrational predissociation (type-II). While type-II dominates at the S1 zero-point level, the type-I quantum yield increases sharply near the S1/S2 conical intersection, which is consistent with the striking dynamic resonance observed for the reactive flux in the proximity of the conical intersection. Nonadiabatic product yield at the asymptotic limit was found to be enhanced predominantly through the type-I channel, suggesting that the quantum-mechanical nature of the reactive flux prepared near the first S1/S2 conical intersection is likely to be retained at the second S0/S2 conical intersection encountered in the later stage as there is little time to be altered during the ultrafast S-CH3 bond extension on the repulsive potential energy curve linking two conical intersections, enabling nonadiabatic control over product yields through the state-selection of the reactive flux.