Optically induced second harmonic generation by six-wave mixing: A novel probe of solute orientational dynamics

The optically induced generation of second harmonics in isotropic media, wh ich arises through a six-wave mixing interaction, is employed in ultrafast studies of orientational relaxation in solution (4-diethylamino-4'-nitrosti lbene 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 ha rmonic frequencies. Time-resolved experiments in nonpolar solvents show tha t the polar order (which supports second harmonic generation) decays as a b iexponential function of time. The two time constants are linearly dependen t on viscosity and fall in the ratio 6:1. This result is shown to be consis tent with a simple model of the six-wave mixing interaction incorporating o rientational diffusion. The two relaxation components arise through contrib utions from the first and third moments of the solute orientational distrib ution function, confirmed by the polarization dependence of the time-resolv ed signal. Thus, it has been demonstrated that odd moments of the orientati onal distribution function, required for a complete description of molecula r orientation, are accessible through six-wave mixing experiments; such dat a are not available in four-wave mixing measurements. The measurements have been extended to polar solvents, where the accelerated population relaxati on 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 mol ecular hyperpolarizability.