Limitations in the Analysis of Vibrational Sum-Frequency Spectra Arising from the Nonresonant Contribution

Sum frequency generation (SFG) spectroscopy is a powerful tool for probing the orientations of molecules at surfaces and interfaces, but oversimplification in the treatment of the nonresonant (NR) contribution has obscured some fundamental limitations in the analysis of SFG spectra. These difficulties are demonstrated for the case of polystyrene thin films. The NR signal invariably distorts the spectrum and can cause changes in the spectra even in the absence of actual structural changes. The NR signal originates not only from the substrate but from all materials in the system and should not be modeled as having a frequency-independent amplitude. Because of its complicated nature, NR signal must be isolated experimentally in order to obtain meaningful results. Suppression of NR signal, however, causes a different type of distortion, due to apodization of the resonant signal in the time-domain. Experimental methods need to be refined to take these limitations into account and to obtain unique spectral parameters to be used for orientation determination.