Infrared studies of the energy gap and electron-phonon interaction in potassium-tetracyanoquinodimethane (K-TCNQ)

Abstract

The polarized reflectance at room temperature of potassium-tetracyanoquinodimethane (K-TCNQ) has been measured. Analysis using the Kramers-Kronig transforms gives the response functions (dielectric function and conductivity) between 300 and 23 000 ${\mathrm{cm}}^{-1\mathrm{}}$. Attempts have been made to fit these functions with Lorentzian oscillators and it has been found that at least two such oscillators were required to give a satisfactory fit to the electronic transitions above 4000 ${\mathrm{cm}}^{-1\mathrm{}}$: a strong, sharp peak corresponding to interband transitions and a weaker, broader absorption at higher energy from an intramolecular excitation. The oscillator-strength sum rule indicates that the energy gap is much larger than that inferred from magnetic resonance measurements and supports the view that Coulomb correlations are important in K-TCNQ. The molecular stretching modes are found to be stronger for the infrared electric field along the chain axis than perpendicular to it. As shown by recent calculations, this effect arises from electron—optical-phonon coupling effects. From the infrared data, we estimate the dimensionless electron—optical-phonon coupling constant to be ${\lambda }_{\mathrm{opt}}\sim 0.1$.