Characterization of the terahertz frequency optical constants of montmorillonite

The real and imaginary parts of the complex index of refraction of montmorillonite between 0.2 THz and 1.4 THz at room temperature are reported. The optical properties of montmorillonite in this frequency range were experimentally determined using time-domain THz spectroscopy. The index of refraction of montmorillonite is n = 2.19 ± 0.02, and the extinction coefficient is k = 0.10 ± 0.01. Distinct frequency dependencies of the index of refraction and the extinction coefficient are not observed within the uncertainties of the measurements. The index of refraction of montmorillonite is modeled using an effective medium theory. The characterization of the optical constants of montmorillonite in the THz-frequency band suggests the potential of time-domain THz spectroscopy for the in-situ investigation of catalytic properties of montmorillonite at the molecular level.