Localized nonadiabatic coupling controls coherent dynamics at conical intersections
Abstract
Conical intersections (CIs) govern ultrafast radiationless relaxation in molecular excitedstates, yet most reduced vibronic models treat interstate coupling as spatially delocalized across the branching space. Here, we investigate how confining the nonadiabatic coupling to a localized region around the CI modifies population transfer, vibrational coherence, and their signatures in two-dimensional electronic spectroscopy (2DES). The system is described by a three-state/two-mode vibronic Hamiltonian containing a ground state and two electronically excited states coupled to explicit tuning and coupling vibrational modes, together with dissipative Drude baths. A Gaussian modulation is introduced into the interstate coupling to compare delocalized, moderately localized, and strongly localized coupling scenarios. Quantum dissipative dynamics are propagated using the hierarchical equations of motion, while waiting-time-dependent 2DES is calculated through the equation-of-motion phase-matching 1 formalism. Localization strongly reshapes the effective excited-state landscape and produces nonmonotonic population-transfer kinetics: intermediate localization yields the fastest transfer, whereas the delocalized model retains the strongest residual oscillations and strong localization suppresses them most efficiently. The calculated 2D spectra reveal pronounced redistribution of diagonal and off-diagonal features as the wave packet traverses the CI region, with localization generating distinct cross-peak growth, sign inversion, and vibronic progression patterns. Peak-resolved dynamics and Fourier analysis identify coherent components near 150, 500, and 1000 cm -1 , while sliding-window transforms demonstrate that localized coupling selectively suppresses low-frequency coupling-mode coherence while preserving or enhancing tuning-mode vibrational signatures. These results establish the spatial extent of nonadiabatic coupling as an important control parameter in CI dynamics.