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

This article introduces two-quantum two-dimensional electronic–vibrational (2Q 2D EV) spectroscopy as a novel experimental approach to directly probe nonadiabatic dynamics at electronic curve crossings in polyatomic molecular systems through vibronic coherences. Using a semi-classical vibronic Hamiltonian model that couples high-frequency (HF) and low-frequency (LF) vibrational modes, the study demonstrates how 2Q 2D EV spectroscopy can isolate coherence transfer signals sensitive to ultrafast electronic relaxation and branching at curve crossings, which are challenging to detect with conventional methods. Simulations of two experimental cases—one with zero delay between pulses and another scanning both delays—show that time- and frequency-domain analyses of the vibronic coherence during the τ2 period reveal characteristic signatures of nonadiabatic dynamics, including frequency shifts and changes in peak intensities. The work highlights the advantages of 2Q 2D EV spectroscopy in accessing unique coherence pathways, reducing background signals, and providing detailed insight into excited state processes such as internal conversion and intersystem crossing, while noting practical considerations like pulse geometry, polarization effects, and transition dipole moment orientations.