EXPRESS: Amplitude-Ratio-Based Reconstruction of Surface Vibrational Spectra for Reducing Reorientation Artifacts in...

Sum-frequency generation (SFG) spectroscopy is a powerful method for probing molecular composition and organization at interfaces. However, interpretation of SFG spectra in multicomponent systems is complicated because spectral intensities depend on both surface population and molecular orientation. This creates a challenge for analysis of the adsorbed population. Two-dimensional correlation spectroscopy (2D-COS) can aid in spectral interpretation, but molecular reorientation can distort spectral trends and lead to incorrect signs of the cross peaks. In this work, we investigate this problem first using model SFG spectra for a binary system in which the population and reorientation contributions are known, and in which the signal changes in a nonlinear manner with respect to the surface mole fraction. We describe an amplitude-ratio-based spectral reconstruction method where fitted vibrational mode amplitudes are used to calculate intra-component amplitude ratios relative to selected anchor modes. The corrected amplitudes are then used to reconstruct SFG spectra with reduced reorientation-induced artifacts. The reconstructed spectra show suppressed homospectral asynchronous features associated with intra-component reorientation and recover the expected heterospectral 2D-COS signs. The method is then applied to experimental SFG spectra of a binary mixture, further demonstrating that amplitude-ratio reconstruction provides a targeted approach for reducing reorientation artifacts and thereby enables population-based interpretation of SFG data.