Using Low-Frequency Vibrational Dynamics to Probe Disorder in Organic Molecular Materials

In organic molecular crystals terahertz radiation interacts with vibrational modes that extend across large domains of a crystal lattice. This makes terahertz spectroscopy a very powerful technique to study the presence and nature of interactions between molecules. By directly exploring the dynamics in the far-infrared, i.e. at relatively low frequencies in the context of vibrational spectroscopy, terahertz spectroscopy is complementary to crystallography techniques that are long-established to resolve the molecular structure of organic crystals: in contrast to the average position of the different atoms that make up the crystal, terahertz spectroscopy can provide crucial insight into the molecular dynamics in condensed matter. Given that the terahertz vibrations correspond to large amplitude vibrations that transverse large areas of the potential energy surface, subtle variations of mass or bond strength have significant impact on the spectral features. As a consequence, terahertz spectra serve as a highly sensitive probe of disorder, which is oftentimes very difficult to resolve with commonly used crystallographic techniques. With increasing disorder the coherent motions, that are characteristic for crystalline systems that exhibit long-range order, collapse into the vibrational density of states which, albeit universal to all disordered materials, still represents the vibrational dynamics of such amorphous materials. By measuring the terahertz spectra as a function of temperature it is possible to determine the onset and extent of molecular mobility in disordered molecular solids. This makes terahertz spectroscopy an ideal complementary technique to a range of scattering techniques, such as neutron inelastic scattering, as well as other spectroscopies such as dielectric spectroscopy.