Fano resonances in the photoinduced H-atom elimination dynamics in the πσ* states of pyrrole

Fano resonances are predicted to be prominent features of the πσ* photochemistry of pyrrole. The Fano interference leads to distorted absorption bands and internally hot photofragments. The interference, studied using high level ab initio theory and quantum mechanics, is supported by two distinct dynamical scenarios controlled by two exit channel conical intersections between the πσ* states and the ground electronic state . For the lowest state ¹A₂(πσ*), the coupling at the conical intersection is weak, and the interfering dissociation pathways are diabatic; for the higher lying ¹B₁(πσ*) state, the coupling at the conical intersection is strong, and the interfering dissociation pathways are adiabatic. Both scenarios persist with increasing number of degrees of freedom included in the dynamic modelling. They are expected to be operative in the πσ* photochemistry of a broad class of model UV biochromophores.