Optical properties of low-dimensional organic conductors with differently oriented conducting layers: (EDT–TTF)$_3$Hg$_2$Br$_6$ and (EDT–TTF)$_3$Hg(SCN)$_3$I$_{0.5}$(PhCl)$_{0.5}$

Spectral optical investigations of two low-dimensional organic molecular conductors with differently oriented conducting layers of ethylenedithiotetrathiafulvalene (EDT-TTF) molecules, namely, the (EDT–TTF)$_3$Hg$_2$Br$_6$ and (EDT–TTF)$_3$Hg(SCN)$_3$I$_{0.5}$(PhCl)$_{0.5}$ single crystals, have been carried out. The polarized reflectance spectra of the single crystals have been measured in the frequency range 700–6500 cm$^{-1}$ (0.087–0.810 eV) at temperatures from 300 to 15 K. The optical conductivity spectra have been obtained using the Kramers–Kronig relations, and their quantitative analysis has been performed in terms of a theoretical model that takes into account electron-electron correlations in the approximation of the Hubbard Hamiltonian for trimerized stacks, the vibronic coupling, and the influence of the counterion on the electronic states in the trimer. A satisfactory agreement between the theoretical and experimental spectra for both crystals made it possible to estimate the parameters of the electronic structure of the crystals in the conducting plane: the integral t of the electron transfer between the EDT-TTF molecules in the trimer, the energy $U$ of the Coulomb repulsion between two electrons (holes) in one EDT-TTF molecule, the electron transfer damping constant $\gamma_e$, the energy shift $\Delta$ of the molecular orbital under the influence of the anions and vibronic coupling, the vibronic coupling constant $g_n$, and the binding energy $E_p$ of the molecular polaron. It has been found that there are large differences in the anisotropies of the optical properties and the obtained Hubbard parameters of the electronic structure for the studied crystals.