  We report ionic strength-dependent phase shifts in second harmonic generation
(SHG) signals from charged interfaces that verify a recent model in which
dispersion between the fundamental and second harmonic beams modulates observed
signal intensities. We show how phase information can be used to unambiguously
separate the chi(2) and interfacial potential-dependent chi(3) terms that
contribute to the total signal and provide a path to test primitive ion models
and mean field theories for the electrical double layer with experiments to
which theory must conform. Finally, we demonstrate the new method on supported
lipid bilayers and comment on the ability of our new instrument to identify
hyper-Rayleigh scattering contributions to common homodyne SHG measurements in
reflection geometries.
