Terahertz disorder-localized rotational modes and lattice vibrational modes in the orientationally-disordered and ordered phases of camphor

The temperature-dependent terahertz spectra of the partially-disordered and ordered phases of camphor (C₁₀H₁₆O) are measured using terahertz time-domain spectroscopy. In its partially-disordered phases, a low-intensity, extremely broad resonance is found and is characterized using both a phenomenological approach and an approach based on ab initio solid-state DFT simulations. These two descriptions are consistent and stem from the same molecular origin for the broad resonance: the disorder-localized rotational correlations of the camphor molecules. In its completely ordered phase(s), multiple lattice phonon modes are measured and are found to be consistent with those predicted using solid-state DFT simulations.