Temperature dependence of light scattering from neat benzene with femtosecond pulses: Are we seeing molecules librate?

We report femtosecond impulsive stimulated scattering experiments on neat liquid benzene at 280 and 343 K. The results are compared with those of Litovitz and co-workers using dynamic light-scattering spectroscopy and found to agree quantitatively. In spite of the evidence for interaction-induced scattering intensities detected at early times, we analyze our results in terms of reorientation of single molecules in the liquid and determine that at temperatures below 310 K, we observe evidence of oscillatory angular velocity correlation functions (AVCF's). These oscillations become more pronounced as the liquid is cooled further, and the time evolution of the correlation indicates that librational motion is predominantly dephased by inhomogeneous mechanisms. We discuss the ubiquitous nature of this finding in many molecular liquids near the crystallization temperature and stress the importance of investigating the fact that MD simulation studies have not generated AVCF's that resemble damped harmonic oscillations, with multiple crossings of the origin, in this thermodynamic regime.