The dielectric constant of TCNQ single crystals as deduced by reflection electron energy loss spectroscopy
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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°. 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 I m ( - l / e ) 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 TT —> 77* dipole-allowed transitions located at about 3.5 and 6.5 eV while, at higher energy loss, the IT + a 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.
I. INTRODUCTION In the last two decades it has been shown that electron energy loss spectroscopy (EELS), in both transmission (TEELS) and reflection (REELS) geometries, can provide a powerful tool to obtain the complex dielectric constant of a material over a very large energy range comparable with the one offered by the Synchrotron Radiation facility.1"5 An electron energy loss spectrum, at least in the range of its first hundred electronvolts, is generally a very complicated superposition of spectral features due to elastic (primary peak) and inelastic events such as surface and bulk scattering arising from the excitation of interband transitions and plasmons. In order to use the experimental loss data to deduce information on the dielectric function of the sample, an accurate deconvolution of all these features is necessary. In this frame, it is easy to understand the importance of any analytical procedure that allows the correct extraction of the pure single scattering bulk contribution proportional to I m ( - l / e ) . The most common separation methods5"10 essentially consist of a set of successive eliminations of the above contributions till the pure single scattering bulk loss function is obtained. However, it must be stressed J. Mater. Res., Vol. 8, No. 10, Oct 1993 http://journals.cambridge.org
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that if it is not particularly difficult to eliminate the elastic peak and the multiple scattering, then it is difficult to remove the surface scattering. This certainly is one of the most important problems, particularly when the related spectral features are not well resolved as single resonances in the spectrum. Surface scattering is very sensitive to experimental conditions such as the angle of incidence and the energy of the primary beam.1'5'11 Varying one or both these conditions, it is possible to modify the surface intensity with respect to the bulk one. Lowering the pr
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