2D correlation analysis in vibrational sum-frequency generation spectroscopy

2D correlation analysis provides a powerful means of extracting structural and dynamical data from spectroscopic experiments. This has been successfully developed for techniques such as IR absorption spectroscopy, where the correlation has additionally been shown to increase the spectral resolution by observing the differing behavior of overlapping absorption bands in response to an external perturbation. Visible-infrared sum-frequency generation (SFG) spectroscopy combines many of the benefits of IR absorption and Raman scattering spectroscopy, and adds a unique structural perspective due to its surface specificity. Bringing the flexibility of 2D correlation analysis to SFG experiments would therefore further enhance the power and utility of this nonlinear vibrational spectroscopy. However, straightforward application of the correlation algorithms to homodyne SFG intensity data is not ideal as the SFG line shape often masks underlying spectral features, resulting in misleading correlation maps. We show that application of correlation analysis to heterodyne SFG experiments restores the qualitative utility of such analyses. An example is provided for the case of leucine adsorption onto surfaces of varying hydrophobicity.