Polarization-resolved ultrafast polarizability relaxation in polar aromatic liquids

The isotropic part of the polarizability relaxation of three polar aromatic liquids has been measured as a function of temperature using a diffractive optic based heterodyne detection method. The isotropic spectral density, which contains only interaction induced components of the liquid's collective polarizability, is contrasted with the anisotropic response to unravel the interaction induced and molecular contributions to the latter. Very similar behavior was observed for the three different aromatic liquids. The fast damping of the isotropic response in the time domain confirms the assignment of the prominent slow exponential relaxation in the anisotropic response to diffusive molecular orientational relaxation. The isotropic response in the frequency domain is found to be asymmetric and peaked at low frequency but extended to higher frequencies, thus contributing over a rather wide frequency range. The relatively low mean frequency of the isotropic spectral density supports the assignment of the highest frequency part of the anisotropic response to molecular librational motion. Finally, a number of affinities are noted between the interaction-induced (isotropic) response and the low frequency shoulder in the anisotropic spectral density, which is so characteristic of the dynamics of aromatic liquids. These data are discussed in relation to some existing molecular dynamics simulations.