Tunnel current through a redox molecule coupled to classical phonon modes in the strong tunneling limit

The tunnel current through a redox molecule coupled to classical phonon modes is calculated in the strong tunneling limit in the Born-Oppenhiemer approximation. The effective "potential energy" of the phonon modes is introduced in the steady state of the in situ scanning tunneling microscope system. The expression for the tunnel current is obtained by using thermal averaging over this potential energy calculated within the spinless model in the wideband approximation. The dependence of the tunnel current on the width of the valence level of the redox molecule, the bias voltage, and the overvoltage is studied and discussed. The results of calculations of the tunnel current and the spectral density are used for interpretation of the experimental data obtained by Tao [Phys. Rev. Lett. 76, 4066 (1996)] and Visoly-Fisher [Proc. Natt. Acad. Sci. U.S.A. 103, 8686 (2006)].