The charge-fluctuating and charge-ordered states in metallic, insulating and superconducting bis(ethylenedithio)tetrathiafulvalene (BEDT-TTF) salts are characterized through the analysis of the infrared (IR) spectral region relevant to the C––C antisymmetric stretch, b1_nu22.This spectral region shows dramatic changes in band intensity and band-shape by varying the temperature. The observed spectral features can be understood on the basis of an extension of the well-known Kubo model for the spectral signatures of an equilibrium between two states. The two-orders of magnitude increase in the IR oscillator strength of the b1_nu22 mode upon ionization has also to be taken into account.
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