For silica substrates, the C/R ratio is 3.6 × 10 7++ 𝑚 + 𝑉 7D in our spectrometer. Interfacial water is an ideal species to probe with nonlinear optics. Even though the nonresonant 2 nd -order hyperpolarizability of water, a , is modest, it is by far the majority species in most aqueous interfacial systems and often aligned in the Stern layer. How an array of water molecules is aligned in the Stern layer is encoded in the second-order susceptibility, a fundamental structural property of matter in noncentrosymmetric environments. It is a measure of how the electrons are distributed in a non-centrosymmetric medium (the interface) and given by the number Ni of a given interfacial species i multiplied by the orientational average of the hyperpolarizability, ai . HD-SHG quantifies c from the measured SHG amplitude and phase according to 19, 20