## Abstract

Experimental measurements of the frequency dependence of the isotropically averaged, molecular dc-induced second-harmonic susceptibility 〈*γ*(−2*ω; ω*, *ω*, 0)〉 and third-harmonic susceptibility 〈*γ*(−3*ω; 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*(−2*ω; ω*, *ω*, 0)〉 and 〈*γ*(−3*ω; 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*(−2*ω; ω*, *ω*, 0)〉 and 〈*γ*(−3*ω; 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 *γ** _{ijkl}*(−

*ω*

_{4};

*ω*

_{1},

*ω*

_{2},

*ω*

_{3}) for shorter chains. Finally, through the detailed comparisons of 〈

*γ*(−2

*ω; ω*,

*ω*, 0)〉 with 〈

*γ*(−3

*ω; 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)}(−

*ω*

_{4};

*ω*

_{1},

*ω*

_{2},

*ω*

_{3}) of glasses by other nonlinear-optical techniques, it is demonstrated that the common reference standard for

*χ*

^{(3)}(−

*ω*

_{4};

*ω*

_{1},

*ω*

_{2},

*ω*

_{3}) of glass is inaccurate, and an improved value is recommended.

© 1991 Optical Society of America

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