Electron Correlation Theory and Experimental Measurements of the Third-Order Nonlinear Optical Properties of Conjugated Linear Chains.

Comprehensive theoretical and experimental studies of the magnitude, sign, dispersion, and length dependence of the third order molecular susceptibility gamma _{ijkl}(-omega_4 ;omega_1, omega_2, omega_3 ) demonstrate that the microscopic origin of the nonresonant third order nonlinear optical properties of conjugated linear chains is determined by the effects of electron correlation due to electron-electron repulsion. Multiple -excited configuration interaction calculations of gamma_{ijkl}(-omega _4;omega_1, omega_2, omega _3) for the archetypal class of quasi-one dimensional conjugated structures known as polyenes reveal for the first time the principal role of strongly correlated, energetically high-lying, two photon ^1 A_{rm g} virtual states in the largest of the two dominant, competing virtual excitation processes that determine gamma _{ijkl}(-omega_4 ;omega_1,omega _2,omega_3). It is also found in studies of the effects of conformation on gamma_{ijkl}( -omega_4;omega _1,omega_2, omega_3) that the origin of the third order optical properties remains basically the same for the all-trans and cis-transoid polyenes, and the results for the two conformations are unified by a common power law dependence of the dominant tensor component gamma_{xxxx}(- omega_4;omega_1 ,omega_2,omega _3) on the physical end-to-end length L of the chain with an exponent of 3.5. Calculations for a noncentrosymmetric conjugated chain demonstrate that virtual excitation processes involving diagonal transition moments that are forbidden in centrosymmetric structures lead to a more than an order of magnitude enhancement in gamma_{xxxx}(-omega _4;omega_1, omega_2,omega _3) compared to the analog centrosymmetric structure. Experimental measurements of the dispersion in the isotropically averaged dc-induced second harmonic susceptibility <gamma( -2omega;omega,omega,0) > and third harmonic susceptibility <gamma(-3omega ;omega,omega ,omega)> in two important polyene structures confirm the electron correlation description of gamma _{ijkl}(-omega_4 ;omega_1,omega _2,omega_3). The measured values of <gamma( -2omega;omega, omega,0)> and <gamma(-3 omega;omega, omega,omega) > at several near infrared fundamental wavelengths for hexatriene, the N = 6 carbon site polyene chain, are in excellent agreement with the calculated magnitude, sign, and dispersion. Corresponding measurements for beta-carotene, a substituted N = 22 polyene chain, together with the results for hexatriene, also verify the calculated power law dependence of <gamma(-omega_4 ;omega_1,omega _2,omega_3) > on L. It was further found in the course of the experimental measurements that the common reference standard for the macroscopic susceptibility chi^{(3)}(-3 omega;omega, omega,omega) is too large by a factor of 2.0, and an improved standard value is proposed.