Development of a two-site Hubbard model for analysis of the electron-molecular vibration coupling in organic charge-transfer salts

Abstract

A two-site Hubbard model has been utilized for the analysis of the intense electron-vibrational bands in the i.r. spectra of organic charge-transfer (CT) salts with dimerized structures. It is shown that intermolecular charge oscillations in totally symmetric anti-phase vibrations are accompanied by a decrease of the energy of the highest occupied two-electron state of the dimer. This effect is responsible for frequency shifts of the electron-vibrational bands compared with relevant Raman active modes and can be described by dimensionless coupling constants. The absorption coefficient for the bands was found to be proportional to the frequency shift. Calculated values are in agreement with the experimental data on BEDT-TTF based monovalent CT complexes.