Frequency dependence of linear and nonlinear optical properties of conjugated polyenes: Anab initiotime-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}(-3{ω};{ω},{ω},{ω})>{γ}(-2{ω};0,{ω},{ω})>{γ}(-{ω};{ω},{ω},-{ω}) >{γ}(-{ω};0,0,{ω}){}{γ}(0;0,{ω},-{ω}).