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

Femtosecond heterodyne detected optical Kerr effect experiments are perform ed on liquid benzene in a wide temperature range. Besides the diffusive ori entational relaxation, at short times an oscillatory behaviour is observed, superimposed to a faster quasi-exponential decay. The spectral densities o btained by Fourier transform are characterised by a broad band in the regio n 0-150 cm(-1), and their profiles show a marked temperature dependence. Si milar 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 lib rating and oscillating in a "solvent" cage; consequently, the width of the observed intermolecular vibrational band has an essentially inhomogeneous o rigin. With the assumption of a bi-modal structure of the frequency distrib ution, a Kubo treatment is able to reproduce the main features of the exper imental spectra at different temperatures. In particular, it is confirmed t hat the peculiar low frequency shape of the spectrum, corresponding to the fast quasi-exponential decay observed in the time-domain experiments, can b e attributed to the motional narrowing effect. The value of the Kubo correl ation 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 te mperature.