The specifics of the VSFG system have been discussed in previous work. Briefly, a Ti: sapphire regenerative amplifier (Spectra Physics-Solstice) with 90 fs pulse duration (2 kHz, 7 W) at 800 nm functions as the pump laser. An optical parametric amplifier (TOPAS Prime) is powered by 70% of this output, which then generates the signal and idler to a noncollinear difference frequency generation (DFG) stage. The remaining 30% is spectrally narrowed to about 10 cm -1 using an air-gap etalon (TecOptics). The beams hit the sample at a 56 • angle, following the Kretschmann method. The IR beam, centered around 3450 cm -1 in the OH stretch region, has an energy of 3 µJ, while the 800 nm picosecond beam has an energy of roughly 10 µJ. For the water spectra measurements, a potential step ranging from -0.1 to -1.0 V against Ag/AgCl was applied to the Au electrode. Each spectrum was gathered by delaying the visible beam relative to the IR beam by approximately 466 fs, with a 2-minute integration time. Further information about the VSFG electrochemical cell can be found in our previous work. This VSFG electrochemical cell uses a backside configuration with plasmonic enhancement, enabling in situ observations of the Au-electrolyte interface at current densities surpassing 1 mA/cm 2 , without the presence of mass transport limitations.