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 w ith two electronic states and a single vibrational coordinate is investigat ed. The two charge-localized diabatic curves have different equilibrium pos ition and the electronic coupling or hopping term (th) is assumed to be a c onstant (PKS model). The solvent effect is simulated by a static random per turbation removing the degeneracy between the left and right charge-localiz ed 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 integ rated) show clearly the signature of such a transition. (C) 2001 Elsevier S cience B.V. All rights reserved.