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

Abstract

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 ( 1 E 0 ) state, oscillations in the electronic population between the S 2 ( 1 E 0 ) and the S 1 ( 1 A 0 1 ) electronic states are observed with a period of $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 0 degeneracy is lifted due to differential coupling to the Jahn-Teller-distorted components of S 2 .

The effects of symmetry and rigidity on non-adiabatic dynamics in tertiary amines: a time-resolved photoelectron velocity-map imaging study of the cage-amine ABCO

“…As discussed above (and as also confirmed by the theoretical work described later) this may be assigned to ionization from a 3p state. For the similar cage-diamine DABCO, initial excitation at 251 nm has been found to give t 1 on a timescale of 1-3 ps, 15,32 where the larger time constant may be explained by the lower excitation energy, as discussed in detail elsewhere. 1 More generally, we note that DABCO (and other TAAs containing two amino groups) have been shown to exhibit additional interactions between the N atom lone pairs (with associated charge delocalization effects).…”

“…1 More generally, we note that DABCO (and other TAAs containing two amino groups) have been shown to exhibit additional interactions between the N atom lone pairs (with associated charge delocalization effects). 15,33,34 This adds additional complexity to the dynamics and so a more detailed direct comparison between DABCO and ABCO is not considered further here. The second, much longer-lived fit component t 2 seen at B2.9 eV arises from 3s ionization once this state is populated via internal conversion.…”

“…7,8 Photophysical properties such as fluorescence, [9][10][11] excimer formation 12,13 and ionization fragmentation 14 have also been reported. The closely related 1,4-diazabicyclo[2.2.2]octane (DABCO) has been studied using time-resolved photoelectron spectroscopy, with the focus on the lonepair-lonepair interaction, 15 which is not present in ABCO.…”

Coherent Electron Hole Dynamics Near a Conical Intersection

“…Our results demonstrate that timeresolved XUV spectroscopy forms a versatile tool for the exploration of correlated dynamics of inner-valence electron holes. This represents an interesting and complex regime distinct from the prior studies of valence electron dynamics in neutral molecules [24][25][26][27]. Furthermore, our ability to quantify the nonadiabatic couplings and quantum coherence opens new avenues for the ultrafast studies of charge and energy transfer mechanisms.…”

Hexamethylcyclopentadiene: time-resolved photoelectron spectroscopy and ab initio multiple spawning simulations

“…This is especially important because a profound understanding of photoinduced dynamics almost always necessitates a detailed comparison of experiment with theory. Many model systems have been investigated by time-resolved experiments [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19] or dynamical simulations. [20][21][22][23][24][25][26] We believe that fundamental questions could be answered more easily by joint studies connecting experimental approaches with such simulations.…”