Frequency dependence of linear and nonlinear optical properties of conjugated polyenes: An ab initio time-dependent coupled Hartree-Fock study

Using a split-valence Gaussian 4-31G basis function augmented with diffuse p and d function and multiconfiguration self-consistent-field optimized geometries, the frequency-dependent polarizability α and second hyperpolarizability γ of ethylene, trans-butadiene, trans-hexatriene, and trans-octatetraene molecules have been calculated by the time-dependent coupled perturbed Hartree-Fock ab initio method. The calculated values of α and γ are in good agreement with the corresponding gas-phase experimental values at the same fundamental wavelength. The various γ values corresponding to different third-order processes exhibit marked difference in their dependence on the conjugation length. For a specific fundamental frequency, the calculated γ value exhibits the following order: γ(-3ω;ω,ω,ω)>γ(-2ω;0,ω,ω)>γ(-ω;ω,ω,-ω) >γ(-ω;0,0,ω)≊γ(0;0,ω,-ω).