Vibronic activation of molecular vibrational overtones in the infrared spectra of charge-ordered organic conductors

A dip-shaped anomaly appearing in the infrared spectrum of charge-transfer organic complexes has been investigated. The anomaly appears at approximately the same frequency (\ensuremath{\sim}2700 cm${}^{\ensuremath{-}1}$), irrespective of light polarization as well as a composition of the complex, when the compounds undergo charge ordering. Isotope-shift measurements for \ensuremath{\theta}-(BEDT-TTF)${}_{2}$RbZn(SCN)${}_{4}$ [BEDT-TTF = bis(ethylenedithio)tetrathiafulvalene] indicates a relationship between the overtone of a C=C stretching mode of the BEDT-TTF molecule and this anomalous signal. Calculations of electron-molecular vibration coupling based on a diatomic molecular dimer model reveals that the overtone is activated by an anharmonicity developed in the adiabatic potential in a charge-separated system. It is presented that numerical calculation based on the simple cluster model reproduces essential features of the experimentally obtained conductivity spectrum.