Synthetic Metals, 19 (1987) 309-316 309 DIMERIZATION, VIBRONIC STRUCTURES, AND OPTICAL GAPS IN THE (TMTTF)2X AND (TMTSF)2X SALTS R. BOZIO, M. MENEGHETTI, C. PECILE and F. MARAN Department of Physical Chemistry, University of Padova 2, Via Loredan, 1-35131 Padua (Italy) ABSTRACT We have analyzed the infrared properties of a I-D dimerized tight binding system with linear coupling to one inter- and one intramolecular phonon. The infrared conductivity has been calculated for a quarter-filled system both in the small U and large U limits. A suitable choice of the model parameters allows the comparison with the experimental infrared conductivity of representative (TMTTF)2X and (TMTSF)2X salts. The results indicate that the infrared properties are quite sensitive to the effects of the gap open by the chain dimerization. This can only be explained if a large on-site correlation U is included. Previously noted correlations between d.c. conductivity and infrared properties are easily explained with the interpretative scheme suggested here. INTRODUCTION The basic physics of the spectroscopic effects of the linear electron intermolecular phonon (e-p) and electron-molecular vibration (e-my) interactions in organic molecular conductors is relatively well understood. Some specific aspects pertaining to systems which have (i) electronic and structural correlations commensurate with the lattice and/or (ii) relatively strong interchain interactions up to real 2D or 3D structures, deserve some further consideration. This is particularly worthwhile since some of the systems which are at the focus of much current attention, namely the TMTCF (C = S or Se) and BEDT-TTF salts, display the above mentioned features. Models which have been used for the analysis of the infrared conductivity and reflectance data of these systems include the so called "dimer charge oscillation" and the "phase phonon" theories Ill. The former has been originally derived for an ensemble of non-interacting molecular dimers and is inadequate for systems which are only slightly dimerized. The latter considers a simplified version of the e-p interactions and does not.account completely for the effect of the commensuration. As a consequence it fails to demonstrate some "selection rules" valid for dimerized systems as well as the predicted band multiplicity in higher order commensurate structures. 0379-6779/87/$3.50 © Elsevier Sequoia/Printed in The Netherlands