Phase-Sensitive Second-Harmonic Generation of Electrochemical Interfaces

Abstract

The interaction between molecular species and charged interfaces plays an indispensable role in a multitude of electrochemical devices. Yet, very little is understood about the nature of this interaction, in particular, the interfacial electric field. Second-order nonlinear spectroscopy such as second-harmonic generation (SHG) can provide chemical information on these interfacial interactions; however, the phase information has received limited attention in electrochemical SHG studies. Here, we demonstrate that the phase of the SHG is essential to the measurement of the electric field at the electrode−electrolyte interface. Our in situ SHG experiment provides strong evidence in support of the parabolic model with complex nonlinear susceptibilities. We conclude that if the absolute phase of the total SHG signal with both χ (2) and χ (3) contributions can be obtained, it would be possible to measure the potential of zero charge of any electrochemical material.

A New Imaginary Term in the Second-Order Nonlinear Susceptibility from Charged Interfaces

“…Improvements in the precision and accuracy of the SHG phase measurement should also be made. In addition to its geochemical relevance, the results presented here may be of interest for nonlinear optical studies of electrochemical, energy-related, and biological interfaces through spectroscopy and microscopy. − Beyond nonresonant conditions, an implication of our findings for the interpretation of resonantly enhanced vibrational SFG spectra of charged interfaces is that even at high ionic strengths, where the DC phase angle, ϕ DC , is near zero, absorptive–dispersive mixing is expected to have a substantial influence on the detected spectral line shapes, as we had alluded to previously. ,, Specifically, depending on the magnitude of the surface potential, the complex-valued iχ X (3) will distort the phases, e iφ i , of each resonant term, ν i , in the χ (2) response from the interface. A correction of the detected χ tot (2) by the χ (3) contribution through eq is thus required, which, in turn, requires knowledge of the frequency dependence of iχ X (3) .…”

“…In addition to its geochemical relevance, the results presented here may be of interest for nonlinear optical studies of electrochemical, energy-related, and biological interfaces through spectroscopy and microscopy. [67][68][69][70][71][72] Beyond non-resonant conditions, an implication of our findings for the interpretation of resonantly enhanced vibrational SFG spectra of charged interfaces is that even at high ionic strength, absorptive-dispersive mixing is expected to render substantial influence on the Ma et al…”

A New Imaginary Term in the 2nd Order Nonlinear Susceptibility from Charged Interfaces

“…In addition to its geochemical relevance, the results presented here may be of interest for nonlinear optical studies of electrochemical, energy-related, and biological interfaces through spectroscopy and microscopy. [67][68][69][70][71][72] Beyond non-resonant conditions, an implication of our findings for the interpretation of resonantly enhanced vibrational SFG spectra of charged interfaces is that even at high ionic strength, absorptive-dispersive mixing is expected to render substantial influence on the detected spectral lineshapes, as we had eluded to earlier. [16][17]20 Applying eqn.…”

Electric Double Layer at the Polycrystalline Platinum–Electrolyte Interface Probed by the Electrokinetic Streaming Current Method

“…The signal strength increased again upon further increase of the potential. The process of determining PZFC of Pt from second-harmonic generation (SHG) is still under development …”

“…The process of determining PZFC of Pt from second-harmonic generation (SHG) is still under development. 24 When no potential is applied on the metal externally (e.g., using a potentiostat), then charging at the interface takes place via the formation of surface complexes mediated by ions in the electrolyte. This is known as site-binding equilibria 25 and is a universal theoretical framework used to explain how the surface charge on solids changes with solution pH or concentration of specifically adsorbing ions.…”