The localization/delocalization dilemma in mixed-valence systems: a theoretical investigation on the application of the photon echo technique

The photon echo (PE) signal from a symmetric mixed-valence system modeled with two electronic states and a single vibrational coordinate is investigated. The two charge-localized diabatic curves have different equilibrium position and the electronic coupling or hopping term (th) is assumed to be a constant (PKS model). The solvent effect is simulated by a static random perturbation removing the degeneracy between the left and right charge-localized states. A set of calculations is performed, keeping fixed the parameters of the model and varying only th, to study the transition from a localized to a delocalized regime. The PE signals (both time resolved and time integrated) show clearly the signature of such a transition.