The 3, 5, 6, and 7 THz resonances of α-glycine

•Temperature dependent terahertz spectra of an α-glycine single crystal reported.•Cryogenic temperatures, bright synchrotron source and a thin sample resolve peaks.•Four modes revealed from 80–240 cm−1, each undergo non-linear redshift with heating.•Bose–Einstein fits show the phonon nature of the far-infrared absorptions.•Two distinct phonon baths responsible for observed low-frequency modes.