Comparison of four-wave mixing techniques for studying orientational relaxation

Three four-wave mixing techniques for measuring orientational relaxation of molecules in solution using time-separated pulses are compared both theoretically and experimentally. This comparison is shown to be greatly facilitated by modifying the conventional third-order response functions to incorporate a portion due to the slowly varying reorientation of the transition dipoles. Ordinary Bragg diffraction of a probe field, in which a transient grating formed by excitation pulses of the same polarization is probed with parallel and perpendicular polarizations, has the advantage of yielding both the time dependence and the magnitude of the induced anisotropy. However, the accompanying acoustic grating interferes with the polarization-sensitive excited state concentration grating unless the probe wavelength falls within a very strong absorption band. The acoustic grating can be eliminated by using perpendicularly polarized pumping pulses (the "crossed grating" configuration). Polarization spectroscopy in which the generated field is collinear with the probe gives the same anisotropy decay kinetics as the crossed grating, but is more sensitive to interference from background birefringence. These three methods, together with time-resolved fluorescence polarization, have been used to examine rotational dynamics in the ground and excited states of rhodamine 6G, 9-aminoacridine, and benzophenone.