General model for the description of the third-order optical nonlinearities in conjugated systems: Application to the all-trans beta-carotene molecule

On the basis of an extended configuration-interaction description of the lowest singlet excited states, we investigate the third-order nonlinear optical response of the beta-carotene molecule by means of the sum-overstates approach. We first uncover the nature of the electronic states contributing primarily to the cubic molecular polarizabilities. From this starting point, we apply a simplified model to evaluate the frequency-dependent third-harmonic generation (THG) and electroabsorption (EA) spectra; the vibronic structure of the spectra is described by taking into account Franck-Condon factors calculated in a displaced oscillator model. For both the THG and EA processes, we obtain good agreement between the theoretical simulations and the experimental data, which underlines the general character of our model.