PULSED TERAHERTZ TRANSMISSION SPECTROSCOPY OF LIQUID CHCL3, CCL4, ANDTHEIR MIXTURES

The frequency-dependent absorption coefficient of CHCl3, CCl4, and the ir mixtures are measured by pulsed terahertz time domain transmission spectroscopy. The absorbance spectrum for neat CHCl3 is shown to compa re well with existing experimental data including coverage of the prev iously difficult to access 0.3-0.9 THz range. Furthermore, fitted curv es to the absorbance spectra of the liquid mixtures, based on mole fra ction weighted sums of the absorption coefficients of pure CHCl3 and C Cl4, indicate the presence of a bulk dipole reducing mechanism, possib ly due to clustering of CHCl3 molecules about CCl4. An algebraic exten sion of the mole fraction weighted fits allows discrimination between relative strengths of the various bimolecular absorption processes. Th e integrated absorption coefficient for collisionally induced absorpti on of CHCl3-CCl4 collisions was found to be less than that for CHCl3-C HCl3 collisions by 2.6 +/- 0.4 THz cm(-1) (integrated absorption coeff icient units). Finally, a new procedure for applying Mori's third-orde r continued fraction to a description of absorption line shapes in liq uids is presented. Attempts to fit the observed absorption spectra to the line shape derived from the third-order truncation of Mori's conti nued fraction were unsuccessful. However, a constrained sum of Mori li ne shapes was found to fit the low and middle frequency portions of th e spectrum reasonably well. This problematic behavior of the Mori anal ysis may not only exemplify nonexponential relaxation of the intermole cular torques, a known problem associated with the third-order truncat ion, but also the existence of two (or more) types of motion (i.e., tr anslations, rotations, and possibly collective motions) causing relaxa tion of the dipolar correlation function. This improvement in the clos eness of the Mori absorbance line shape fir to the experimentally dete rmined data illustrates the possibility of straight forward extraction of dynamical properties of liquids from absorbance spectra. This theo ry provides an analytical, yet limited, alternative to the more compli cated but more comprehensive determination of dynamical properties obt ained through molecular dynamics simulations.