Terahertz time-domain spectroscopy of crystalline amino acids

Absorption spectra of some amino acids in crystalline phase in a terahertz (THz) frequency range (< 100 cm^-1) were investigated by coherent time-domain spectroscopy. Most of amino acids show characteristic absorption peaks. Some of those peak frequencies show strong and extraordinary temperature dependence, which is difficult to explain by anharmonisity of a single potential alone. We show that the temperature dependence of the peak frequencies is well explained by a model based on double-well potentials of the normal modes.