Terahertz Time-Domain Spectroscopy of Ionic Liquids

Abstract

We investigated the terahertz (THz) permittivity of ionic liquids by the THz time-domain spectroscopy. Ionic liquids show very broad permittivity spectra in the THz region, representing the existence of ultrafast dielectric responses. The permittivity in the THz region cannot be expressed by the Drude model, which explains permittivity spectra based on the motion of free charged particles. The THz responses of ionic liquids are, therefore, cannot be understood by the simple collective motion of free ions. The analysis based on the Debye model also failed in explaining the THz permittivity and the far-infrared absorption spectra of the ionic liquids simultaneously. This result suggests that intermolecular damped oscillations and/or dielectric relaxations including correlation between motions of ion molecules are necessary for representing the ultrafast dynamics of the ionic liquids.