Electron delocalization in asymmetric trimeric mixed-valence clusters

The statistical distribution of electron density in a C_2v one-electron trimeric mixed-valence cluster is considered in terms of the Piepho-Krausz-Schatz vibronic model using the adiabatic approximation. Conditions are revealed under which the lower branch of the adiabatic potential shows three minima, each corresponding to predominant localization of the electron on one of the three ionic centers. It is shown that when the symmetry of the system is disturbed, the potential has two minima, with the system localized on one of two equivalent centers at either minimum. Conditions are considered under which these minima merge into one minimum, which corresponds to delocalization of the electron over a pair of ions, i.e., partial delocalization. For a D_3h symmetric trimer, it is demonstrated that in the region of transfer parameters and vibronic couplings, where the localized and completely delocalized states coexist, even with insignificant distortions there are only three localization minima, and the central minimum, which corresponds to a completely delocalized state, vanishes.