Vibronic structure in the "metallic" reflection band of the TCNQ⁰ crystal

Abstract

Polarized reflection spectra off the (010) face of the first singlet transition of crystalline tetracyanoquinodimethane (TCNQ0) have been measured at room and liquid helium temperatures. The spectrum consists of a massive block of high reflectivity, corresponding to a stop band 1.4 eV wide, with well defined edges at 2.8 and 4.2 eV. The reflectivity of the long wavelength edge undergoes a large increase upon cooling, and vibrational structure appears between 2.8 and 3.1 eV due to freezing out of powerful polariton scatterers, the intramolecular vibrations. All structure is assoicated with volume polaritons since it is not shifted when the crystal is coated with a layer of frozen gas. The complex dielectric function, calculated by a Kramers-Kronig transformation, shows two regions where the real part ε1 is negative. The spectra provide a test of the basic notions of strong vibronic coupling in molecular crystals. Polariton scattering by lattice phonons is weak, wherea scattering by intramolecular modes is much stronger. Copyright © 1977 American Institute of Physics.