Collective vibrational spectra of a- and y-glycine studied by terahertz and raman spectroscopy

Terahertz time-domain spectroscopy (THz-TDS) is used to investigate the absorption and dispersion of polycrystalline α - and γ -glycine in the spectral region 0.5-3.0 THz. The spectra exhibit distinct features in these two crystalline phases. The observed far-infrared responses are attributed to intermolecular vibrational modes mediated by hydrogen bonds. We also measure the Raman spectra of the polycrystalline and dissolved glycine in the frequency range 28-3900 cm^-1. The results show that all the vibrational modes below 200 cm^-1 are nonlocalized but are of a collective (phonon-like) nature. Furthermore, the temperature dependence of the Raman spectra of α -glycine agrees with the anharmonicity mechanism of the vibrational potentials.