Analysis of the Terahertz Absorption Spectrum of Melamine

In this paper, with terahertz (THz) time-domain spectroscopy, we measure the characteristic absorption peaks of melamine and analyze their vibrational modes with the help of solid-state density functional theory (DFT) calculations. The observed strong absorption peaks at 2, 2.27, and 2.61 THz are well reproduced but show red shifts compared to the theoretical results. Detailed normal mode analyses are carried out, which reveals that those THz peaks are generated from intermolecular vibrations. Temperature-dependent THz measurements suggest that this red shift mainly comes from the temperature effect. Thus, three peaks are assigned to the calculated intermolecular vibrational motions. This result indicates that THz spectroscopy can be a useful tool to detect the details of intermolecular structures.