Temperature dependence of the dielectric function of C6H6(I) and C6H5CH3(I) measured with THz spectroscopy

We report on an experimental investigation of the temperature dependence of the intermolecular dynamics in liquid benzene and toluene. With the use of THz time domain spectroscopy we measured the complex dielectric function (0.2-3.3 THz) of the liquids, at temperatures between -6 degrees C and 75 degrees C. By analyzing the dielectric loss (as opposed to the absorption coefficient) we found three contributions to the dielectric function for toluene and two for benzene. In the dipolar liquid toluene we observed a contribution from rotational diffusion at lower frequencies in addition to the two high-frequency librations also observed in benzene. The temperature and density dependence were different for the two librational bands, probably due to the different effect of three-body interactions for the two processes. Furthermore, we present measurements of the low-frequency depolarized Raman spectra as a function of temperature for benzene and toluene. These have been compared with the dielectric loss at similar temperatures. Moreover, for benzene a similar comparison is made between the dielectric loss and the reduced Raman susceptibility obtained by optical Kerr effect spectroscopy. Libration of the symmetric top axis is found in all spectra. (C) 2000 American Institute of Physics. [S0021-9606(00)51433-0].