Double-quantum two-dimensional electronic–vibrational spectroscopy: Theory. Vibronic coherences in nonadiabatic phenomena

Nonadiabatic phenomena are ubiquitous in polyatomic molecular systems and are responsible for many ultrafast dynamics occurring outside of the Born-Oppenheimer approximation. In particular, electronic curve crossings offer ultrafast, sub-100 fs pathways for efficient electronic relaxation between potential energy surfaces, where the electronic and vibrational degrees of freedom become strongly coupled. Due to the unique mixture of temporal and spectral resolution required to detect electronic curve crossings, experimental observation has remained a considerable challenge. Here, double-quantum coherence two-dimensional electronic-vibrational (2Q 2D EV) spectroscopy is introduced for the first time and proposed as an experimental approach to monitor vibronic dynamics near and at electronic curve crossings directly through vibronic coherences using a mixture of broadband visible and infrared pulses. A semi-classical vibronic Hamiltonian is used that characterizes the parametrically defined coupling between high-frequency vibrations and low-frequency vibrational modes involved in tuning electronic potential energy surfaces. This work displays how unique multidimensional pulse sequences can uncover dynamics that are hidden in conventional techniques.