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Determination of Surface Preference using Heterospectral Surface--Bulk Correlation Spectroscopy.

We outline a method by which the surface preference of species in a multi-component mixture may be obtained using surface-specific visible-infrared sum-frequency generation (SFG) spectroscopy combined with bulk infrared absorption and/or Raman data. In general, the problem is complicated by the fact that the SFG signal is a function of both the surface coverage and the structure of the molecules. Two-dimensional correlation analysis can be used to reveal which spectral features are changing synchronously, i.e. in phase with each other, and which ones are evolving in a manner that is phase-shifted by 90° (asynchronous correlation) as a function of the bulk composition. We provide a framework for determining surface preference from correlations between vibrational modes in SFG spectra, and between modes from SFG and bulk IR and/or Raman spectra. When compared to the equivalent analysis performed using SFG spectra alone, this method can be used with data obtained using a single beam polarization, and in congested spectral regions where fitting to isolate the behavior of individual vibrational modes is not robust.