In the experiments, we begin with a ten-nanometer thin nickel layer (5.1 Å ± 0.5 Å rms roughness) prepared by physical vapor deposition on a glass microscope slide that is subsequently placed into a custom-designed spectro-electrochemical cell (please see Supplementary Information Fig. , and Methods section) connected to an electrochemical workstation. Probing with a femtosecond laser oscillator (80 fs, 1034 nm, 75.5 MHz) and employing single photon counting, we record the SHG intensity simultaneously with the current density as a function of applied potential at pH 13 (as well as pH 7, 9, and 11) and 1 M ionic strength (NaClO4 as well as alkali chlorides). The SHG intensity is quadratic in input power (Supplementary Information Fig. ). We find SHG intensity minima that precede the peak potentials of the well-known Ni 2+ /Ni 3+ redox pair (Fig. ), which follows the expected (scan rate) 1/2 dependence (Supplementary Information Figure ) .