Pulsed Terahertz Transmission Spectroscopy of Liquid CHCl₃, CCl₄, and their Mixtures

The frequency-dependent absorption coefficient of CHCl 3, CCl 4, and their mixtures are measured by pulsed terahertz time domain transmission spectroscopy. The absorbance spectrum for neat CHCl 3 is shown to compare well with existing experimental data including coverage of the previously difficult to access 0.3−0.9 THz range. Furthermore, fitted curves to the absorbance spectra of the liquid mixtures, based on mole fraction weighted sums of the absorption coefficients of pure CHCl 3 and CCl 4, indicate the presence of a bulk dipole reducing mechanism, possibly due to clustering of CHCl 3 molecules about CCl 4 . An algebraic extension of the mole fraction weighted fits allows discrimination between relative strengths of the various bimolecular absorption processes. The integrated absorption coefficient for collisionally induced absorption of CHCl 3 −CCl 4 collisions was found to be less than that for CHCl 3 −CHCl 3 collisions by 2.6 ± 0.4 THz cm -1 (integrated absorption coefficient units). Finally, a new procedure for applying Mori's third-order continued fraction to a description of absorption line shapes in liquids is presented. Attempts to fit the observed absorption spectra to the line shape derived from the third-order truncation of Mori's continued fraction were unsuccessful. However, a constrained sum of Mori line shapes was found to fit the low and middle frequency portions of the spectrum reasonably well. This problematic behavior of the Mori analysis may not only exemplify nonexponential relaxation of the intermolecular torques, a known problem associated with the third-order truncation, but also the existence of two (or more) types of motion (i.e., translations, rotations, and possibly collective motions) causing relaxation of the dipolar correlation function. This improvement in the closeness of the Mori absorbance line shape fit to the experimentally determined data illustrates the possibility of straightforward extraction of dynamical properties of liquids from absorbance spectra. This theory provides an analytical, yet limited, alternative to the more complicated but more comprehensive determination of dynamical properties obtained through molecular dynamics simulations.