THE DIELECTRIC-CONSTANT OF TCNQ SINGLE-CRYSTALS AS DEDUCED BY REFLECTION ELECTRON-ENERGY-LOSS SPECTROSCOPY

The dielectric constant of tetracyanoquinodimethane (TCNQ) single crystals has been obtained by reflection electron energy loss spectroscopy (REELS) over the 0-60 eV energy range, using primary electron energies ranging from 0.5 to 1.5 keV at an incidence angle of about 40-degrees. A self-consistent method is discussed concerning the evaluation of the surface and bulk contributions to the loss spectra. As a result, for the first time, the Im(- 1/epsilon) function and the dielectric constant of TCNQ have been deduced in such a wide energy range. According to the results obtained by other authors, the low-energy loss spectral profile is characterized by two main structures ascribed to the pi --> pi* dipole-allowed transitions located at about 3.5 and 6.5 eV while, at higher energy loss, the pi + sigma plasmon, centered at about 21.5 eV, dominates the spectrum. The differences among the spectra taken at different primary energies are interpreted as due only to surface effects, more evident in the low-energy-loss spectral region. The results are in good agreement with those obtained by recent transmission-mode (TEELS) experiments.