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

Area of Science:

• Quantum chemistry
• Molecular dynamics
• Spectroscopy

Background:

• Conical intersections are critical points in molecular potential energy surfaces where electronic states become degenerate.
• Vibrational dynamics play a crucial role in nonadiabatic transitions at these intersections.
• Understanding these dynamics is key to controlling photochemical reactions and energy transfer processes.

Purpose of the Study:

• To investigate the role of vibrational dynamics in the vicinity of conical intersections.
• To elucidate the mechanisms driving nonadiabatic transitions in excited electronic states.
• To identify specific vibrations responsible for modulating electronic state coupling.

Main Methods:

• Time-resolved photoelectron spectroscopy (TRPES) was used to monitor electronic population dynamics.
• Ab initio computations were employed to characterize excited states and conical intersections.
• Analysis of conical intersection topography identified key vibrational modes.

Main Results:

• Oscillations in electronic population between S2 and S1 states of DABCO were observed with a ~3 ps period.
• Rapid nonadiabatic dynamics were detected, despite weak coupling typically associated with Rydberg states.
• Specific vibrations were identified that maximally modulate the coupling between the S1 and S2 electronic states.

Conclusions:

• Strong electronic state coupling in DABCO occurs near conical intersections, driven by specific vibrational modes.
• The observed population oscillations are attributed to vibronic coupling and the lifting of degeneracy in the S2 state.
• This study provides insights into controlling nonadiabatic dynamics through targeted vibrational excitation.