Single crystal optical properties of two forms of tetrathiafulvalenium iodide (TTF-In)

Polarized specular reflectance measurements over the energy range 0.6-5 eV are reported for two crystalline forms of tetrathiafulvalenium iodide (TTF-In). Since the plane of each TTF molecule is either approximately parallel or perpendicular to an external face of each crystal, polarized reflectance measurements can sample the various symmetry-allowed electronic transitions of the TTF molecule. The optical constants have been determined by means of a Lorentzian oscillator fit. A Drude-like edge is seen for light polarized along the conducting axis of monoclinic TTF-In (n∼0.7). Structure in the reflectance data for light polarized perpendicular to the stacking axes in both forms of TTF-In is interpreted as being primarily due to in-plane transitions of the TTF+ ion. Transitions polarized along the long and short axes of the TTF+ ion in orthorhombic TTF-In (n=2) can be resolved to aid in the assignment of the transitions for the TTF+ ion.