Non-Born-Oppenheimer wavepacket dynamics in polyatomic molecules: vibrations at conical intersections in DABCO

The role of vibrational dynamics in the vicinity of conical intersections is investigated using the first two electronically excited states of 1,4-diazabicyclo[2,2,2]octane (DABCO) by combining time-resolved photoelectron spectroscopy with ab initio computation. Upon resonant excitation of the origin band of the short-lived S-2 (E-1') state, oscillations in the electronic population between the S-2 (E-1') and the S-1 ((1)A(1)') electronic states are observed with a period of similar to 3 ps. Ab initio computations are employed to characterise these low-lying excited states, which arise from single excitations into the 3s and 3p Rydberg orbitals. Although Rydberg states are generally only weakly coupled, DABCO exhibits rapid nonadiabatic dynamics. This implies that strong coupling occurs only in the immediate vicinity of a conical intersection, enabling unique identification of those vibrations which generate the nonadiabatic transitions. To this end, seams of conical intersection are located at energetically relevant geometries, engendered by differential distortions of the S-1 and S-2 potentials due to vibronic coupling and a Jahn-Teller-distorted S-2 minimum energy point. From an analysis of the conical intersection topography, those vibrations leading to a maximal modulation of the coupling between the electronic states are readily identified. The observed oscillation in the decay of S-2 state population is thereby assigned to the beat frequency between two sets of vibronic eigenstates within the S-1 manifold, coherently prepared together with another set at the S-2 band origin, and whose nominal e' degeneracy is lifted due to differential coupling to the Jahn-Teller-distorted components of S-2.