The optically induced generation of second harmonics in isotropic media, which arises through a six-wave mixing interaction, is employed in ultrafast studies of orientational relaxation in solution (4-diethylamino-4‘-nitrostilbene in a range of solvents). It is shown that polar order can be induced in the samples by simultaneous irradiation at the fundamental and second harmonic frequencies. Time-resolved experiments in nonpolar solvents show that the polar order (which supports second harmonic generation) decays as a biexponential function of time. The two time constants are linearly dependent on viscosity and fall in the ratio 6:1. This result is shown to be consistent with a simple model of the six-wave mixing interaction incorporating orientational diffusion. The two relaxation components arise through contributions from the first and third moments of the solute orientational distribution function, confirmed by the polarization dependence of the time-resolved signal. Thus, it has been demonstrated that odd moments of the orientational distribution function, required for a complete description of molecular orientation, are accessible through six-wave mixing experiments; such data are not available in four-wave mixing measurements. The measurements have been extended to polar solvents, where the accelerated population relaxation of the solute is apparent. The dynamics at early times in polar solvents are complex, an effect which is explained in terms of a time-dependent molecular hyperpolarizability.
|Number of pages||7|
|Journal||The Journal of Physical Chemistry A|
|Publication status||Published - 1 May 1999|