Molecular Spectroscopic Aspects of the Third Order Susceptibility

The third order electric dipole susceptibility χ ⁽³⁾, (-ω_d, ω_a, ω_b, ω_c) is enhanced when combinations of frequency arguments fall near material resonances. The use of a physical process which depends on the magnitude of this function allows spectroscopy to be performed. We have selected four wave mixing (4WM) as a method to simultaneously observe Raman and two-photon (TP) resonances. In (partially degenerate) 4WM the presence of intense oscillating electric fields creates nonlinear polarizations. The component at frequency ω₃ = 2ω₁ - ω₂ is Under phase matched conditions P ⁽³⁾ (ω₃) acts as a coherent source to generate light intensity at the same frequency. Because this light derives coherence from the ω1 and ω2 laser beams, even low ω3 intensities are easily detected after frequency and spatial filtering. In this paper we present  the first applications of these techniques in molecular solids and vapors. The latter appears elsewhere and will not be fully discussed.