Anharmonicity-driven redshift and broadening of sharp terahertz features of α-glycine single crystal from 20 K to 300 K: Theory and experiment

© 2020 Elsevier B.V. For the first time, large single crystals of the simplest amino acid, glycine, have been used to determine the temperature dependence of its terahertz spectrum. High-quality spectra with very sharp absorption features are observed at cryogenic temperatures. The α-glycine structure and the purity of the crystals were verified via Raman spectroscopy and X-ray diffraction. Spectral redshift with increasing temperature was observed for all absorption bands in the terahertz region (10–250 cm−1, or 1–8 THz) over the temperature range of 20–300 K. X-ray diffraction revealed expansion in all planes of the crystal lattice over the same temperature range. A Bose-Einstein distribution was used to model the frequency position shift of the two lowest-energy fundamental modes at 50 cm−1 and 69 cm−1. On this basis, we attribute the observed redshift and broadening with increasing temperature to the anharmonic potential associated with the phonon bath.