Double Quantum Two-Dimensional Electronic-Vibrational Spectroscopy - A Vibronic Perspective of Conical Intersections.

Nonadiabatic phenomena are ubiquitous in polyatomic molecular systems and are responsible for many ultrafast dynamics that are not well defined within the Born-Oppenheimer approximation. Conical intersections offer ultrafast, sub-100 femtosecond pathways for efficient electronic relaxation between potential energy surfaces, where the electronic and vibrational degrees of freedom become strongly coupled. The temporal and spectral resolution required to detect conical intersections has made their experimental observation considerably challenging. We propose using double quantum coherence two-dimensional electronic-vibrational (2Q 2D EV) spectroscopy as an experimental approach to monitor vibronic dynamics around a conical intersection, using a mixture of broadband visible and infrared pulses. Here, we develop a semi-classical 3-level system model Hamiltonian that characterizes the coupling between high- and low-frequency vibrational modes that are involved in the conical intersection formation. This Hamiltonian is used to simulate 2Q 2D EV spectra of a photoexcited system passing through a conical intersection. We demonstrate 2Q 2D EV spectroscopy as a helpful tool in identifying nonadiabatic phenomena using vibronic coherences.