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

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Abstract
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 absorptionspectrum, 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.