Nonlinear optical responses of molecular systems with vibronic coupling in fluctuating environments

The topological profiles of the nonlinear optical properties of a two-level molecular system involving simplified versions of intramolecular coupling immersed in a thermal reservoir have been studied in four-wave mixing (FWM) spectroscopy. Solvent effects have been explicitly considered by modeling the nonradiative interaction with the solute as a random variable. To formulate a solution, it is necessary to introduce stochastic considerations. We assume that system–solvent interactions induce random shifts in the Bohr frequency of the adiabatic states, and their manifestation corresponds to the broadening of the upper level. The Born–Oppenheimer (BO) electronic energy curves for this molecular model consist of two intertwined harmonic oscillator potentials shifted in both their position and their minimum equilibrium energy. The critical quantities for this analysis are the transition and permanent dipole moments, which depend strongly on intramolecular coupling. Our results show how absorption and refractive index are affected by vibronic coupling and solvent stochasticity.