Terahertz molecular science in condensed phases
The terahertz (THz) frequency region is located between the microwave and infrared regions. Molecular motions and intermolecular interactions in the THz region reflect unique properties of materials, which are important to understand structural stabilization, chemical reactions, and functionalities of molecular systems. We have been applying spectroscopic methods in the THz region to various problems in the research field of molecular science. By using THz time-domain spectroscopy (TDS), we measure the response of materials an electromagnetic field. Molecular crystals show vibrational structures even in the THz region at low temperatures, and mode assignment is performed by solid-state density functional theory and crystal structures obtained by X-ray diffraction. The agreement between the experiment and theoretical calculation depends on how we treat the intermolecular interactions in the calculations. Consideration of London dispersion force is crucial to explain the vibrational spectra in the THz region. The normal modes are generally a mixture of intermolecular and intramolecular vibrational modes. Hydrogen bonding liquids such as water interact with electromagnetic wave in a broad range from the MHz to mid-IR region. Liquid water has different types of molecular motion associated with hydrogen bond dynamics such as librations, intermolecular stretching mode, and collective reorientational relaxation, and these dynamics have specific time-scales and frequencies. In order to understand the dynamics of water in the THz region, we perform broadband dielectric spectroscopy ranging from MHz to mid-IR region. The obtained complex dielectric spectra are analyzed in terms of a model function for time-correlation function of the total dipole moment of the system. This work is also extended to proteins to investigate the effects of thermal excitation and hydration on the protein dynamics. Charge carriers in the condensed materials also interact with THz waves efficiently. The examples are conductive polymers and organic semiconductors.
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