Dimer approach in examination of electron-molecular vibration coupling in BEDT-TTF based charge-transfer salts

Summary form only given. The dimer approach has been utilized for the analysis of fundamental vibrations in spectra of BEDT-TTF based monovalent charge-transfer salts. It was shown that each vibrational level of molecule in the dimer splits into two ones corresponding to the in-phase and anti-phase dimeric modes. Frequencies as well as selection rules for the in-phase vibrations remain the same as those for the isolated molecule; the second anti-phase component appears in the spectra due to changes in selection rules and looks like an additional set of vibronic bands. Last bands should reveal themselves in i.r. as well as in Raman spectra and are the result of reduction of symmetry of the dimerized system in comparison with the regular one caused by an intermolecular interaction. Dimer coupling constants indicate how many additional anti-phase modes are valid and may be observed in the experiment. The selection rules for allowed coupling constants are derived as direct product of the irreducible representation corresponding to the dimeric vibrations and this for the intermolecular interaction (controlled by symmetry of atoms or valence bonds of adjacent molecules that are supposed to be interacting). The calculation of the dimer normal vibrations let us to reproduce positions of intensive vibronic bands recorded in the i.r. spectra of the monovalent salts and to estimate the strength of the intermolecular interaction in the compound. The analysis shows also that the coupling in the BEDT-TTF based charge transfer salts takes effect not only on the totally symmetric a/sub g/ modes but also on the vibrations of other symmetry species.