  Second-order nonlinear spectroscopy has proven to be a powerful tool in
elucidating key chemical and structural characteristics at a variety of
interfaces. However, the presence of interfacial potentials may lead to
complications regarding the interpretation of second harmonic and vibrational
sum frequency generation responses from charged interfaces due to mixing of
absorptive and dispersive contributions. Here, we examine by means of
mathematical modeling how this interaction influences second-order spectral
lineshapes. We discuss our findings in the context of reported nonlinear
optical spectra obtained from charged water/air and solid/liquid interfaces and
demonstrate the importance of accounting for the interfacial
potential-dependent \c{hi}(3) term in interpreting lineshapes when seeking
molecular information from charged interfaces using second-order spectroscopy.
