Phase-Resolved Spectra of Charged Aqueous Solution Interfaces from High-Resolution Broadband Sum Frequency Generation Vibrational Spectroscopy (HR-BB-SFG-VS)

Phase-resolved sum frequency generation (SFG) spectra provide direct access to the complex nonlinear optical response of interfaces, which can be obtained either through direct phaseresolved measurement or by analyzing intensity spectrum. Recently, we demonstrated that phase-resolved SFG spectra of the neutral air/neat water interface can be obtained from highresolution broadband SFG intensity spectra using multi-peak Lorentzian function fitting (MLFF) with physical constrain with the knowledge of the absolute phase for the ‘free’ O-H peak. Here, we further extend this MLFF approach to charged air/aqueous interfaces of the anionic surfactant sodium dodecyl sulfate (SDS) and the cationic surfactants cetyltrimethylammonium bromide (CTAB). Sub-1 cm-1 and high Signal-to-Noise ratio (SNR) high-resolution broadband SFG vibrational spectroscopy (HR-BB-SFG-VS) intensity spectra of the SDS and CTAB aqueous solution interfaces were acquired in the 2400-4400 cm-1 region and analyzed using a phase-inclusive multi-peak Lorentzian line-shape fitting. Since the C-H region spectra from the interfacial surfactant layer are with excellent spectral resolution and high SNR, they provided the internal phase reference to determine the phase of the whole spectra, including the O-H spectral features originated from the interfacial and bulk contributions of the water species. We demonstrate in this report that the whole phase-resolved spectra of the SDS and CTAB aqueous solution interfaces thus obtained are consistent with previously reported phase-sensitive SFG measurements. These results suggest that HR-BBSFG-VS intensity spectra with high-SNR can provide a practical and reliable route for obtaining phase-resolved spectra of the charged interfaces, forming reliable basis for future quantitative analysis and understanding of complex interfaces in general.