Optical properties of organic conductor and semiconductor crystals: Model for a half-filled dimerized chain

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Abstract
The interpretation of the charge transfer and vibronic optical spectra of molecular organic half-filled crystals is shown to be possible on the basis of a periodic cluster model made up of four sites. The results obtained by using this model, which considers an extended Hubbard Hamiltonian and the interaction of intramolecular vibrations with the electronic system, are reported in detail to show the dependence of the various calculated excitations on the intermolecular dimerization, a structural distortion characteristic of the compounds on which attention is focused. The effect of including off-diagonal Coulomb interaction is also considered. The available experimental spectra of a well-known half-filled compound like K-TCNQ (where TCNQ is tetracyanoquinodimethane) are satisfactorily fitted and the values of the parameters obtained by the fittings are used to suggest an interpretation of the phase transition of this compound. Some parallel observations for half-filled polymers are also reported.