We collect a fringe on the uncoated glass side and obtain a fitted phase, jfit, using a trigonometric fit function detailed in Supplementary Information Section S2. This fitted phase is the offset we apply to the phases we obtain when moving the cell such that the laser beam hits the nickel nanolayer. Replicate measurements obtained by assembling and reassembling 15 glass/nickel slides in our cell show jnickel:airjglass:air = -76° ± 19° (standard deviation obtained from Gaussian peak potential vs (scan rate) 1/2 and linear fits (y=a+b*x) where a=0.5259 ± 0.0009 V and 0.497 ± 0.003 V and b=0.542 ± 0.006 mV 1/2 s 1/2 and -0.47 ± 0.02 mV 1/2 s 1/2 for the anodic and the cathodic peaks, respectively. ), but for c (2) and F(0)tot. The c (2) F(0)tot pair from the main text is given as well (empty circles). d) Second-order nonlinear susceptibility and total interfacial potential as a function of applied potential for electrolyte held at pH 7, 9, 11, and 13, indicated by increasingly darker line color. All pH runs performed on the same electrode; pH was changed using a peristaltic pump. Inset: F(0)tot at +0.9V applied vs pH, and slope of 0.09 ±0.04 V pH -1 from weighted linear least squares fit (solid line).