The fast dynamics of benzene in the liquid phase. Part I. Optical Kerr effect experimental investigation

Femtosecond heterodyne detected optical Kerr effect experiments are performed on liquid benzene in a wide temperature range. Besides the diffusive orientational relaxation, at short times an oscillatory behaviour is observed, superimposed to a faster quasi-exponential decay. The spectral densities obtained by Fourier transform are characterised by a broad band in the region 0-150 cm(-1), and their profiles show a marked temperature dependence. Similar behaviour is observed in a 1 : 6 molar solution of benzene in carbon tetrachloride. The experimental observations can be interpreted by assuming that the basic microscopic system, able to account for the main dynamical properties of the liquid at short times, consists of a benzene molecule librating and oscillating in a "solvent" cage; consequently, the width of the observed intermolecular vibrational band has an essentially inhomogeneous origin. With the assumption of a bi-modal structure of the frequency distribution, a Kubo treatment is able to reproduce the main features of the experimental spectra at different temperatures. In particular, it is confirmed that the peculiar low frequency shape of the spectrum, corresponding to the fast quasi-exponential decay observed in the time-domain experiments, can be attributed to the motional narrowing effect. The value of the Kubo correlation time tau (c) thus estimated ranges between 0.70 and 0.25 ps on going from temperatures close to the benzene melting point to near the boiling temperature.