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 CC antisymmetric stretch, b1ν22. 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 b1ν22 mode upon ionization has also to be taken into account.
BEDT‐TTF CC antisymmetric stretch, b1ν22.