High-resolution polarized far-infrared vibrational spectra of semiconducting TTF-TCNQ and TSeF-TCNQ

High-resolution polarized far-infrared spectra of TTF-h4-TCNQ-h4, TTF-h4-TCNQ-d4, TTF-d4-TCNQ-h4, TTF-d4-TCNQ-d4, and TSeF-TCNQ are presented. These have been obtained mainly by a bolometric technique, which is possible only at low temperatures, but some measurements of transmission through aligned mosaics of single crystals at various temperatures are also reported. The bolometric technique is described and its limitations and advantages demonstrated. The measured wavenumber shifts enabled us to assign most of the many TTF-TCNQ features. For [Formula: see text], a strong group is believed to be due to Rz librations, which should be optically inactive, and a quartet has been assigned to the TTFν10(au) mode (a torsion of the two fulvalene rings about the central bond), which should be very weak. The anomalous infrared activity of both of these modes indicates that they are probably large components of the Peierls distortion. Normally inactive gerade intramolecular modes of both molecules are also observed at low temperatures. In TSeF-TCNQ, all of the "activated" modes involve only the TSeF molecule, indicating that the Peierls distortion produces little displacement or rotation of the TCNQ molecule. Estimates of the TSeF-TCNQ intrinsic and impurity energy gaps are also given.