Phonon-Induced Electron–Hole Excitation and ac Conductance in Molecular Junction

We investigate the linear ac conductance of molecular junctions under a fixed dc bias voltage in the presence of an interaction between a transporting electron and a single local phonon in a molecule with energy ω0. The electron–phonon interaction is treated by the perturbation expansion. The ac conductance as a function of the ac frequency ωac decreases or increases compared with the noninteracting case depending on the magnitude of the dc bias voltage. Furthermore, a dip emerges at ωac ∼ 2ω0. The dip originates from the modification of electron–hole excitation by the ac field, which cannot be obtained by treating the phonon in the linear regime of a classical forced oscillation.