Dipole Correlation Functions in Liquid Benzenes Measured with Terahertz Time Domain Spectroscopy

We report an experimental investigation of the dynamics of the dipole correlation functions in selected liquid benzenes (C6H12, C6H6, C6D6, C6H3F3, and C6F6). Using terahertz time domain spectroscopy, we have measured the complex frequency response, i.e., both index of refraction and absorption, in the range from 0 to 2.5 THz (0 to 83 cm-1), where the response is dominated by collective collision-induced processes. We have analyzed the data using the Mori memory function formalism and obtained dipole correlation function dominated by a simple exponential decay for all the substituted benzenes. The observed decay times are shorter than typical reorientation times and proportional to the free rotation time of the molecules. This indicates that small angle librational motion is responsible for the dipolar absorption observed.