Use of Variable Time-Delay Sum-Frequency Generation for Improved Spectroscopic Analysis

The interpretation of vibrationally resonant sum-frequency generation (VR-SFG) spectra is often complicated by two factors: spectral congestion and the presence of a nonresonant signal. With broadband VR-SFG systems, the spectra are further distorted because of incomplete sampling, or apodization, of the resonant free induction decay (FID) in the time domain. An experimental method is presented that takes advantage of these time-domain effects to obtain more accurate parameters than can be achieved by fitting a single VR-SFG spectrum. VR-SFG spectra are acquired at multiple delay times of the visible pulse with full suppression of the nonresonant signal and then simultaneously fit to determine a single set of spectral parameters. The multiple, independent spectra serve to constrain the results of the fitting. This variable time-delay approach allows for improved determination of the parameters of the resonant spectrum, including the proper number of peaks in the spectrum, and is demonstrated for thin films of polystyrene and surface-bound octadecylsilane. An additional advantage of the technique is that it can be implemented with minimal modification to an operational broadband VR-SFG system.