Theoretical study of the initial non-radiative 1 B_u → 2 A_g transition in the fluorescence quenching of s-trans-butadiene: Electronic structure methods and quantum dynamics

The photodynamics of s-trans-butadiene in the 6 eV excitation energy range is investigated by ab initio quantum dynamical methods, paying particular attention to the nonadiabatic coupling between the 1B_u and 2A_g singlet excited states. The existence of a conical intersection between their potential energy surfaces is confirmed. Key parameters of the system, like the energy gap between the interacting states and their coupling strength, are critically assessed. Up to eight nuclear degrees of freedom are considered in the dynamical treatment and are shown to lead to a more realistic description of the interactions. The gas phase (jet) UV absorption spectrum is well reproduced. The related ultrafast nonradiative population transfer from 1B_u to 2A_g is the initial processes leading to fluorescence quenching of trans-butadiene.