In We expect that our fundamental insights will add to the ongoing rapid development of molecular electrochemistry. Noting that the level of quantification presented here would not be possible with the information and models that have existed until now, the experimental data we present can serve as benchmarks for theoretical models of the electrical double layer and electrochemistry. They establish that the Stern layer water molecules flip prior to the onset of the oxygen evolution reaction and thereby open the possibility to pursue the energy barrier for water flipping as a means for addressing the OER's high overpotential on nickel anodes. Beyond the new physical insights and experimental benchmarks, the ability to 1) count the number of Stern layer water molecules, 2) determine their net absolute orientation, and Speelman and Geiger page S2 x 10 -3 A s, corresponding to 7 x 10 15 to 8 x 10 15 electrons transferred (comparable to undoped NiO electrodes) (2) and thus 7 to 8 monolayer equivalents of Ni 2+ /Ni 3+ , or an electrochemically active oxide thickness of 1.4 to 1.6 nm, assuming a Ni-O bond length of 2Å. This oxide thickness is on the order of we reported earlier from atom probe tomography for iron nanolayers (3) prepared using the same low-impurity PVD method that is employed here for the nickel nanolayers. Atomic force microscopy (Bruker ) shows the electrodes to have a root mean square roughness of 5.1 Å ± 0.5 Å over 1 x 1 µm 2 and 300 x 300 nm 2 areas before and after the electrochemical measurements (please see Supplementary Information Figs S6), matching that of the substrates onto which they are deposited. Optical imaging shows no pinholes, but they are found on rare occasions in SEM images (please see Supplementary Information Figs S7). Water contact angles recorded immediately after nanolayer formation are <10° but increase to over 50° when the nanolayers are left in ambient laboratory air for several hundred hours (please see Supplementary Information Figs S8), indicating hydrocarbon buildup. We therefore subject each electrode, once mounted in the e-chem cell, to replicate cyclic voltammograms (-0.3 V to +0.8V) until they are indistinguishable from one another, typically five to ten cycles. We direct 0.2 W from a LightConversion Flint oscillator (model FL1-02) producing 80 femtosecond pulses at 1034 nm at a 75.5 MHz repetition rate onto the electrode:electrolyte interface using a defocused (-1 cm) 10 cm lens (spot size ca. 100 µm diameter) and block the reflected fundamental light from the external air:window interface. We direct the SHG signal pulses along with the reflected fundamental pulses from the electrode:electrolyte interface towards an off-axis parabolic mirror and a 0.5 mm thin uncoated calcite time delay compensator (Newlight Photonics, CAL12050-A) to account for spatial and temporal dispersion at the detector, as described in our earlier work . The fundamental and SHG pulses are then sent through a 1 mm thin fused silica phase shifting unit (Edmund Optics) on a rotating stage (Standa model 8MR174- Speelman and Geiger page , and then through a 50 um thin z-cut a-quartz wafer (Precision Micro-Optics PWQB-368252) producing the local oscillator (LO). The SHG pulse pair (signal+LO) then interferes at the detector (Hamamatsu H8259-01) as a function of the phase shifting unit (PSU) angle.