Diffusive and oscillatory dynamics of liquid iodobenzene measured by femtosecond optical Kerr effect

The dynamics of liquid iodobenzene are studied by means of time resolved heterodyne detected optical Kerr effect in a wide temperature range (263-371 K). In the picosecond regime the relaxation is characterized by a biexponential decay, attributed to the rotational diffusion of an anisotropic rotator. The temperature dependence of the two relaxation times agrees only in part with the prediction of the hydrodynamic theory. The subpicosecond dynamics is essentially oscillatory in nature; the Raman spectra obtained by Fourier transform of the time domain data show the contribution of two intramolecular low-frequency vibrations, and that of the intermolecular dynamics. The intermolecular spectra at different temperatures are interpreted on the basis of the Brownian oscillator model, and consist of the superposition of overdamped and underdamped modes. The intermolecular spectrum of the liquid shows a close resemblance with the low-frequency Raman spectrum of crystalline iodobenzene, and suggests an interpretation in terms of collective dynamics with coupling of librational and translational oscillations. The evolution with temperature of the spectra indicates that at high temperature the time-independent picture of the intermolecular modes is not adequate. (C) 1999 American Institute of Physics. [S0021-9606(99)51717-0].