Electronic excitations in α-(BEDT-TTF) 3(NO 3) 2

Abstract

The indicatrix orientation, the transmission spectra polarized parallel to the principal axes, and the birefringence were measured in small crystals of α-(BEDT-TTF) 3(NO 3) 2 in the NIR and VIS spectral range. The results are discussed with regard to the orientation of the BEDT-TTF (ET) molecule axes and the arrangement of the intermolecular S-S, S-o and C-0 contact directions. The principal polarizabilities are calculated by composing the polarizabilities along the ET molecule axes and along the several contact directions. The measured large rotation of the indicatrix by 50° about the b axis (monoclinic crystal) results from different excitations. The dispersion of the measured extinction angle can be reproduced by calculations using decreasing polarizabilities in the NIR due to the anomalous dispersion of the ET-anion contact excitations and using increasing polarizabilities along the ET molecule axes due to intramolecular excitations lying in the VIS. The dispersion of the birefringence and the absorption spectra confirm this behavior.