Optical properties of cesium tetracyanoquinodimethanide,Cs2(TCNQ)3

Abstract

Room-temperature polarized reflectance measurements have been made on cesium tetracyanoquinodimethanide, ${\mathrm{Cs}}_2$ ${\mathrm{}\left(\mathrm{TCNQ}\right)\mathrm{}}_3$, over the frequency range between the far infrared and the near ultraviolet. The optical properties of the compound were obtained by Kramers-Kronig analysis. These properties are dominated by vibrational features at low frequencies and by electronic excitations at high frequencies. The observed vibrational features include ordinary intramolecular modes and "anomalous" infrared activity of the totally symmetric vibrations. This latter absorption results from the interaction of these vibrations with the unpaired electron on the TCN${\mathrm{Q}}^{-}$ ion. Two electronic excitations are observed for the electric field polarized along the TCNQ chains. The excitation higher in frequency is attributed to an intermolecular charge transfer from one TCN${\mathrm{Q}}^{-}$ ion to an adjacent ion, while the lower-frequency excitation results from a charge-transfer transition to an adjacent neutral molecule. Estimates for the effective on-site Coulomb repulsion energy $U$ of 1.14 eV, and for the transfer matrix element $t$ of 0.17 eV, are obtained from these data. Experimental values for the electron-molecular-vibration coupling constants are also obtained.