Nonlinear optical processes in short polyenes: Configuration interaction description of two-photon absorption and third-harmonic generation

We use a multireference determinant single-double configuration interaction approach within a Pariser-Parr-Pople Hamiltonian (based on long-range hopping integrals and the use of the Ohno formula) to investigate linear polyenes containing from four to sixteen carbons. We calculate the low-lying excited states, the two-photon absorption spectrum, and the third-harmonic generation (THG) response. The mA_g state, essential to the cubic nonlinear optical response, is found to saturate as the 6A_g state when chain length increases; for the longer polyenes, another high-lying A_g state also becomes important. We analyze the length dependence of the static third-order susceptibility χ⁽³⁾; indication of the beginning of saturation behavior is found. Focusing our attention to the two-photon resonance peak present in the free-electron laser THG measurements on polyacetylene, we conclude that the experimental data can be explained within the strongly correlated electron one-dimensional model used in this work, in addition to the weakly interacting model successfully exploited in a previous study.