Mechanisms of in situ scanning tunnelling microscopy of organized redox molecular assemblies

theoretical frame for in situ electrochemical scanning tunneling microscopy (STM) of large adsorbed redox molecules is provided. The in situ STM proce ss is viewed as two consecutive interfacial single-step electron transfer ( ET) processes with full vibrational relaxation between the steps. The proce ss is therefore a cycle of consecutive molecular reduction and reoxidation. This extends previous approaches where resonance tunneling, or coherent si ngle-channel ET, were in focus. The dependence of the tunneling current on the bias voltage and overvoltage is calculated when both transitions are ei ther fully adiabatic or fully diabatic, and when one transition is fully ad iabatic and the other one fully diabatic. A particular feature of the fully adiabatic limit is that each oxidation-reduction cycle is composed of mani folds of individual interfacial ET events at both electrodes, enhancing ele ctron tunneling significantly compared to single-ET. The voltage dependence s 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 cohere nt tunneling where the maximum is shifted approximately by the nuclear reor ganization Gibbs free energy. Recent data for in situ STM of iron protoporp hyrin IX on highly oriented pyrolytic graphite (Tao, N. J. Phys. Rev. Left. 1996, 76, 4066-4069) show such a maximum and therefore accord well with se quential two-channel ET. This shows that multiphonon ET theory extended to in situ STM of redox molecules offers a comprehensive frame where distincti on between different tunneling mechanisms is feasible.