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A theoretical frame for in situ electrochemical scanning tunneling microscopy (STM) of large adsorbed redox molecules is provided. The in situ STM process is viewed as two consecutive interfacial single-step electron transfer (ET) processes with full vibrational relaxation between the steps. The process is therefore a cycle of consecutive molecular reduction and reoxidation. This extends previous approaches where resonance tunneling, or coherent single-channel ET, were in focus. The dependence of the tunneling current on the bias voltage and overvoltage is calculated when both transitions are either fully adiabatic or fully diabatic, and when one transition is fully adiabatic and the other one fully diabatic. A particular feature of the fully adiabatic limit is that each oxidation−reduction cycle is composed of manifolds of individual interfacial ET events at both electrodes, enhancing electron tunneling significantly compared to single-ET. The voltage dependences show spectrocopy-like features. Particularly, the overvoltage dependence has a maximum at the equilibrium potential when the potential distribution in the tunnel gap is symmetric. This is different from resonance and coherent tunneling where the maximum is shifted approximately by the nuclear reorganization Gibbs free energy. Recent data for in situ STM of iron protoporphyrin IX on highly oriented pyrolytic graphite (Tao, N. J. Phys. Rev. Lett. 1996, 76, 4066−4069) show such a maximum and therefore accord well with sequential two-channel ET. This shows that multiphonon ET theory extended to in situ STM of redox molecules offers a comprehensive frame where distinction between different tunneling mechanisms is feasible.
Kuznetsov et al. (Sat,) studied this question.