Assigning the characteristics of an ATR dove prism for use with terahertz frequencies: Supplemented with molecular dynamic simulations

In this paper we characterize an in house attenuated total reflection (ATR) sample cell unit, for use with a table top Terahertz-Time domain system (THz-TDS) spectrometer. The custom designed ATR unit offers several key advantages compared to conventional transmission spectrometry. Solid, liquid and powdered samples can easily be prepared for analysis and are easily removed and accessed without the need of dismantling the sample cell. Multiple reflections between the windows in transmission cells are no longer present with the ATR setup. If required the prism can be removed and replaced with a more traditional transmission cell without interfering with the spectrometers optics and optical beam path. The ATR unit was tested with a series of water-isopropyl alcohol (IPA) mixtures, to measure the performance and sensitivity of the ATR unit. The series of mixtures ranged from 0-100% IPA with increments of 10%. The ATR unit was shown to define each increment, with a steady and linear drop of in THz absorbance with added IPA. The real and imaginary parts of the refractive index and therefore, dielectric constants were also calculated at each sample interval. The refractive index increased linearly at the measured THz range from 0.5 and 1 THz from a refractive index of 1 at pure IPA to a maximum of 3 at pure water. To further our understanding of the ATR results, molecular dynamic simulations of the IPA mixtures were performed using an OPLS/AA force field. These simulations produced a series of trajectories from which the auto correlated Fourier transform was calculated from the total dipole moment magnitude of each frame. This gives us the spectral density of each IPA-water mixture which is directly comparable to the THz spectrum experimentally seen with the ATR unit. These simulations repeated the same pattern showing a decrease in THz absorbance with increasing IPA solute. From these results, we conclude the ATR unit is an effective tool in terahertz spectroscopy. The agreement with the simulated results gives us confidence the ATR unit is performing as designed.