Collective vibrational spectra of α- and γ-glycine studied by terahertz and Raman spectroscopy

Terahertz time-domain spectroscopy is used to investigate the absorption and dispersion of polycrystalline alpha- and gamma-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 cl-glycine agrees with the anharmonicity mechanism of the vibrational potentials.