Molecular dynamics of β‐carotene in solution by resonance enhanced optical Kerr effect

The orientational dynamics of β‐carotene in n‐alkane solutions is investigated by resonance enhanced optical Kerr effect. By use of this spectroscopic technique, it is possible to selectively investigate the relaxation of a probe molecule at a concentration level low enough to allow the observation of the averaged single‐molecule dynamics. For delay times longer than ∼20 ps all solutions show a single exponential decay, with a time constant depending on the viscosity, that is ascribed to the β‐carotene orientational relaxation. The dependence on viscosity of the measured relaxation times is compared with the predictions of different models. The purely hydrodynamic theories overestimate, by far, the solute effective volume and hence its orientational relaxation time; a much better agreement is obtained from two quasihydrodynamic models.