ORIENTATIONAL CORRELATION-FUNCTIONS AND POLARIZATION SELECTIVITY FOR NONLINEAR SPECTROSCOPY OF ISOTROPIC MEDIA .1. 3RD-ORDER

The contribution of orientational relaxation to the tensor components of the third-order nonlinear polarization is evaluated for off-resonan ce Raman and dipole resonant experiments in the perturbative limit. Or ientational correlation functions are calculated within the model of o rientational diffusion for all third-order tensor components relevant to isotropic media. General expressions for polarization geometries th at are selective to particular components of the signal, i.e., magic a ngles, are derived for collinear and crossed-beam excitation geometrie s. It is shown that although limited selectivity exists for Raman spec troscopies, no combination of polarizations will give complete selecti vity to the isotropic, anisotropic, or nonresonant contributions to th e Raman polarizability tenser. For resonant spectroscopies, the four-t ime correlation function that describes the orientational polarization decay can be written as the product of three two-time correlation fun ctions. While magic angles for orientational relaxation will exist for experiments that probe population dynamics, such as pump-probe and tr ansient grating spectroscopies, orientational relaxation cannot be rem oved from coherence experiments such as the photon echo. (C) 1996 Amer ican Institute of Physics.