Ultrafast Double Optical Pump Technique for Probing Charge Transfer Processes at the Electrode-Electrolyte Interface
Authors: Lapointe François , Tong Yujin , Campen R. Kramer 
Published: February 2020

The electrode-electrolyte interface is one of the most technologically relevant there is, as it is where charge transfer occurs in driven electrochemical reactions. The short times after heterogeneous charge transfer witness the occurrence of primordial processes, such as intramolecular rearrangement and solvation shell reorganization. Yet, probing the evolution of this interface with ultrafine time resolution (10-15—10-12 s) is highly challenging, if not impossible, with electronics-based techniques. [1] Photoinjection techniques have been used to trigger charge capture by acceptors at the electrode-electrolyte interface, which was observed with > 10-9 s time resolution from photocurrent. [2] We aim to use the aqueous electron as a test bed for probing the heterogeneous charge transfer with ultrafine time resolution. The ultrafast kinetics of this fundamental species have been studied in the bulk [3], but it was never observed at the interface.
We have used ultrafast laser pulses in the ultraviolet (UV) range with the intent of triggering photoinjection from a 200 nm thick polycrystalline gold electrode to the 0.5 M Na2SO4 electrolyte. The energy of the ultraviolet photons (4.64 eV) is high enough for excited electrons to reach the LUMO of water, and exposure of the electrode to the UV laser beam indeed creates a large photovoltage. A second ultrafast pulse of lower energy (ranging from 0.31 to 1.85 eV) is used to resonantly pump the excitation after a controlled delay varying from -4 ps to 100 ps with the smaller steps being of 50 fs. This second pump pulse also causes a photovoltage change, but smaller by an order of magnitude than the photovoltage caused by the first. The photovoltage due to the second pump pulse decays at a rate that is dependent on the photon energy, which yields a resonant profile that matches the absorption profile of hot electrons in water during the solvation process. [4] For instance, the decay rate at 0.31 eV is very fast, being comparable or smaller than our 50 fs minimum step size, and is independent of pump power. At 1.85 eV, however, the photovoltage lasts for longer than 100 ps, showing that a long-lasting species endures at the gold-electrolyte interface. In between, we capture the smooth evolution of the decay kinetics.
We expect this technique to enable the study of ultrafast kinetics of relevant processes directly at the electrode-electrolyte interface, giving valuable mechanistic insight on important electrochemical reactions such as the oxygen reduction reactions (ORR).
References
[1] Amatore & Maisonhaute (2005) Anal Chem 77:303A
[2] Babenko et al. (1977) J Electroanal Chem 76:347
[3] Silva et al. (1998) Phys Rev Lett 80:1086
[4] Migus et al. (1987) Phys Rev Lett 58:1559
Figure: (a) Schematics of the double pump experiment. A pulse of UV light first arrive at the electrode and photoinjects electrons into the electrolyte. After a controlled delay, a pulse of lower energy interacts resonantly with the excitation. (b) Extracted decay of the open circuit potential for a 2nd pump pulse at 1.85 eV as a function of delay.

Figure 1