Single crystal growth and polarization absorption spectroscopy of theophylline anhydrous for terahertz vibrational mode assignment

In an attempt to provide a procedure for mode assignment in the terahertz (THz) frequency range, we fabricated an apparatus for single crystal growth via a temperature difference method to provide organic crystals that are sufficiently thin for wide-range THz transmission spectroscopy.Single-crystal theophylline anhydrous (TPAH) was successfully fabricated. THz polarization spectroscopy measurements were performed on TPAH crystal samples at 70 K. Assignment of the absorption peaks was carried out by comparing measurement results with those from density functional theory (DFT) calculations under periodic boundary conditions.