Nonadiabatic Vibronic Dynamics at a Conical Intersection with Morse Potentials

Conical intersections (CIs) are primary pathways for ultrafast nonradiative relaxation in photoexcited molecules, yet their spectroscopic signatures often reflect a complex interplay of electronic motion, vibrational dynamics, and environmental dephasing. This challenge becomes particularly significant when the nuclear coordinate driving the relaxation is strongly anharmonic, since harmonic approximations commonly employed in CI models cannot capture the asymmetric level structure and wave-packet evolution associated with realistic bondstretching motion. Here, we develop a dissipative two-state, two-mode vibronic model that combines an anharmonic Morse tuning coordinate with a harmonic coupling coordinate and a three-level extension for simulating two-dimensional electronic–vibrational (2DEV) spectroscopy. Reduced density-matrix dynamics are propagated using the hierarchy equations of motion, while absorptive 2DEV spectra are calculated within a phase-matching formalism. To isolate the effect of Morse anharmonicity, we further introduce a parameter-matched fully harmonic reference model with the same local frequencies, potential-surface displacements, electronic coupling, bath parameters, temperature, and initial conditions. The resulting potential-energy surfaces generate an asymmetric branching space in which the same nonadiabatic dynamics appear as rapid population transfer in the adiabatic representation and as a more gradual redistribution in the diabatic representation. The calculated dynamics exhibit coherent oscillations with a dominant component near 620 cm⁻¹, while the 2DEV spectra reveal time-dependent redistribution of vibronic intensity during passage through the intersection region. Unequal vibronic peak spacings and evolving center-line slopes emerge as experimentally testable signatures of anharmonic vibronic coupling and changing electronic-vibrational correlation within the present model. These results demonstrate that anharmonic Morse-based models qualitatively alter how CI dynamics appear in 2DEV spectroscopy.