Effects of charging and polarization on molecular conduction via the source-sink potential method

The current-voltage relationships of butadiene and octatetraene are computed using the source-sink potential method with self-consistent Huckel theory. Molecular orbital resonances appear as steps in current, considerably broader than the resonances in the transmission spectrum at any specific bias. This broadening is due primarily to the charging of the molecule as bias increases. A perturbation theory based model is derived to account for the observations. In the case of octatetraene, the HOMO resonance manifests at high voltage when the HOMO energy is raised to the Fermi level of the plus electrode due to the charging of the LUMO and LUMO + 1 levels. Specifically, the LUMO and LUMO + 1 steps in current are augmented by portions of the HOMO contribution to current.