Broadband models and their consequences on line shape analysis in vibrational sum-frequency spectroscopy

Vibrational sum-frequency generation (SFG) spectroscopy can provide valuable qualitative and quantitative information about molecular species at surface and buried interfaces. For example, the resonance frequency of a particular chemical function group is revealing of the surface environment, especially when compared to what is observed in bulk IR absorption or Raman scattering spectra. Furthermore, the amplitude of the mode can be related to the molecular orientation, providing a detailed quantitative account of the surface structure. Each of these attributes, however, requires fitting the spectra to some vibrationally resonant line shape. This is particularly challenging when the modes of interest co-exist with broad resonance features, such as water O-H stretching. In this perspective, we examine the merits and consequences of different approaches to fitting homodyne SFG data. We illustrate that, while any model can provide a useful description of the data, no model can accurately and consistently provide even the relative phase deeply encoded in homodyne data without the use of additional information.