Thermoreflectance of Tetrathiafulvalinium Tetracyanoquinodimethanide (TTF-TCNQ)

We report the first observation of a marked temperature dependence in the optical properties of TTF-TCNQ, based on thermoreflectance spectroscopy. The structure just above the plasma edge exhibits an abrupt 25% drop with $\overrightarrow{\mathrm{e}}\parallel \overrightarrow{\mathrm{b}}$ as the temperature is lowered past 45-50 K; simultaneously, a strong response for this transition with $\overrightarrow{\mathrm{e}}\perp \overrightarrow{\mathrm{b}}$ emerges. This supports the suggestion that a librational distortion of the TCNQ ion may be responsible for a Peierls transition in the complex.