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 he terodyne detected optical Kerr effect in a wide temperature range (263-371 K). In the picosecond regime the relaxation is characterized by a biexponen tial decay, attributed to the rotational diffusion of an anisotropic rotato r. The temperature dependence of the two relaxation times agrees only in pa rt with the prediction of the hydrodynamic theory. The subpicosecond dynami cs is essentially oscillatory in nature; the Raman spectra obtained by Four ier transform of the time domain data show the contribution of two intramol ecular low-frequency vibrations, and that of the intermolecular dynamics. T he 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 liqui d shows a close resemblance with the low-frequency Raman spectrum of crysta lline iodobenzene, and suggests an interpretation in terms of collective dy namics with coupling of librational and translational oscillations. The evo lution with temperature of the spectra indicates that at high temperature t he time-independent picture of the intermolecular modes is not adequate. (C ) 1999 American Institute of Physics. [S0021-9606(99)51717-0].