Probing Charge Transfer at the Metal/Aqueous Solution Interface using Resonant Two Photon Photovoltage Spectroscopy

The phenomenological Butler-Vollmer and the symmetric or asymmetric Marcus-Hush theory have been rather successful in prediction of the electron transfer rate across the solid/liquid interfaces. However, the former provides no microscopic insights and the latter assumes that the timescale of solvent structural fluctuation dynamics (solvent vibrations) is well separated from that of electron transfer dynamics. As a result its applicability to systems where there is strong interaction between the electrode and the reactant/product , and thus the timescales may be similar, is limited. Such systems are of particular importance in many catalytic reactions.

While important, experimental study of these elementary processes are extremely challenging. One needs to create a vibrational excitation at an electrode/electrolyte interface at a well-defined time and probe the associated current or voltage change as this excitation thermalizes. Because vibrational lifetimes in aqueous solution are typically 0.01-10s of picoseconds this implies that current or voltage resolution on these timescales is required. Such time scales are not possible by conventional electrochemical techniques.

In this study we create imperfect, monocomponent Ferrocene-terminated alkane thiol self-assembled monolayers (SAMs) on Au and place them inside a thin-film spectroelectrochemical cell kept at open circuit conditions. To explore the relationship of particular vibrations to charge transfer, we electronically excite the Ferrocene with a femtosecond pulse in the visible and vibrationally excite the system using a femtosecond pulse in the mid-infrared in resonance with the aromatic C-H stretch vibrations. The resulting two photon photovoltage (2PPV) signal is then measured as a function of delay between the infrared and visible pulses. We find the Ferrocene coupled surface state is electronically excited, and relaxes, on a timescale ~120 fs. However this electronically excited state relaxes to a nonequilibrium ground state electronic structure under open circuit conditions, Ferrocenium, whose subsequent relaxation back to Ferrocene can be induced by aromatic CH stretch excitation leading to an additional 4 ps timescale in the photovoltage response. Interestingly the photon energy of the CH stretch, ~0.4 eV, is 1/2 that estimated for the perfect, bicomponent, Ferrocene terminated alkane thiol SAM. To our knowledge these results are the first measurement of mode-specific charge transfer at an electrode-electrolyte interface. Because the technique is quite general, and the type of reactions where this effect is likely to occur are ubiquitous in electrocatalysis, we expect it to be of wide interest.

The program can be found here.