Validation of Spectra and Phase in Sub‑1 cm^–1 Resolution Sum-Frequency Generation Vibrational Spectroscopy through Internal Heterodyne Phase-Resolved Measurement

Abstract

Reliable determination of the spectral features and their phases in sum-frequency generation vibrational spectroscopy (SFG-VS) for surfaces with closely overlapping peaks has been a standing issue. Here we present two approaches toward resolving such issue. The first utilizes the high-resolution and accurate line shape from the recently developed subwavenumber high-resolution broadband SFG-VS (HR-BB-SFG-VS), from which the detail spectral parameters, including relative spectral phases, of overlapping peaks can be determined through reliable spectral fitting. These results are further validated by using the second method that utilizes the azimuthal angle phase dependence of the z-cut α-quartz crystal, a common phase standard, through the spectral interference between the SFG fields of the quartz surface, as the internal phase reference, and the adsorbed molecular layer. Even though this approach is limited to molecular layers that can be transferred or deposited onto the quartz surface, it is simple and straightforward, as it requires only an internal phase standard with a single measurement that is free of phase drifts. More importantly, it provides unambiguous SFG spectral phase information on such surfaces. Using this method, the absolute phase of the molecular susceptibility tensors of the CH₃, CH₂, and chiral C–H groups in different Langmuir–Blodgett (LB) molecular monolayers and drop-cast peptide films are determined. These two approaches are fully consistent with and complement to each other, making both easily applicable tools in SFG-VS studies. More importantly, because the HR-BB-SFG-VS technique can be easily applied to various surfaces and interfaces, such validation of the spectral and phase information from HR-BB-SFG-VS measurement demonstrates it as one of the most promising tools for interrogating the detailed structure and interactions of complex molecular interfaces