Experimental measurements of the frequency dependence of the isotropically averaged, molecular dc-induced second-harmonic susceptibility 〈γ(−2ω; ω, ω, 0)〉 and third-harmonic susceptibility 〈γ(−3ω; ω, ω, ω)〉 of conjugated linear-chain structures are in excellent agreement with the results of many-electron, multiple-excited-configuration-interaction calculations. For hexatriene, the N = 6 carbon site linear polyene chain, we demonstrate the first successful comparison, to our knowledge, between experiment and theory for the magnitude, sign, and dispersion of 〈γ(−2ω; ω, ω, 0)〉 and 〈γ(−3ω; ω, ω, ω)〉. These results provide experimental verification of the importance of electron-correlation effects in the third-order nonlinear-optical properties of conjugated structures. Measurements of 〈γ(−2ω; ω, ω, 0)〉 and 〈γ(−3ω; ω, ω, ω)〉 for β-carotene, the intermediate length, N = 22, conjugated chain, are also reported, and the frequency dependence is well described by a three-level model that is developed based on the results of the electron-correlation description of γijkl(−ω4; ω1, ω2, ω3) for shorter chains. Finally, through the detailed comparisons of 〈γ(−2ω; ω, ω, 0)〉 with 〈γ(−3ω; ω, ω, ω)〉 and of experiment with theory, as well as through examination of previously reported values of the macroscopic third-order susceptibility χ(3)(−ω4; ω1, ω2, ω3) of glasses by other nonlinear-optical techniques, it is demonstrated that the common reference standard for χ(3)(−ω4; ω1, ω2, ω3) of glass is inaccurate, and an improved value is recommended.
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