(1) (9.6 ± 1.9 × 10 7++ m + V 7+ from off-resonant SHG experiments at the air/water interface or 10.3 × 10 7++ m + V 7+ estimated from the third-order molecular hyperpolarizability obtained through quantum mechanical calculations). The thirdorder response from the water molecules in the EDL is modulated by the phase, 𝜑 is the wavevector mismatch and 𝜆 2 the Debye screening length at a given bulk ionic strength, computed from Debye Hückel theory. For our experimental geometry, ∆𝑘 8 is 1.1 x 10 7 m -1 . The Debye length for each of our experimental conditions of ionic strength is determined using Debye-Hückel theory using bulk water's relative permittivity of 78. We recently reported an additional purely imaginary third-order contribution that may be of quadrupolar nature, 𝑖𝜒 9 , where,