Nonlinear infrared and optical responses of a Holstein-Peirls-Hubbard dimer

A simple model based on the Holstein-Peierls-Hubbard Hamiltonian has been used to calculate the nonlinear responses at infrared and optical frequencies. The model is applied to a molecular ion radical dimer in order to account for the contribution to the nonlinear responses arising from the intermolecular charge transfer excitations and from their coupling to both intramolecular and intermolecular phonons. A similar calculation has been performed on a model quadrupolar conjugated molecule characterized by intramolecular charge transfer excitations. The calculations are performed according to a collective electronic oscillator scheme by solving the Liouville equation for the bielectronic density matrix. Such a choice allows us to retain the ability, inherent in the Hubbard type models, to fully account for the on-site electron correlation effects. Calculated spectra are reported for one-photon and two-photon absorption (the latter in the form of the imaginary part of the Kerr susceptibility) and for third harmonic generation. Narrow resonances are observed in the infrared, related mostly to the coupling with intramolecular modes. The off-resonant contribution to the nonlinear susceptibility arising from the electron-phonon interactions appears to be marginal (in the order of 1%) in all cases.