Measuring Complex Sum Frequency Spectra with a Nonlinear Interferometer.

Currently, the only techniques capable of delivering molecular-level data on buried or soft interfaces are the nonlinear spectroscopic methods: sum frequency generation (SFG) and second harmonic generation (SHG). Deducing molecular information from spectra requires measuring the complex components-the amplitude and the phase-of the surface response. A new interferometer has been developed to determine these components with orders-of-magnitude improvement in uncertainty compared with current methods. Both the sample and reference spectra are generated within the interferometer, hence the label nonlinear interferometer. The interferometer configuration provides experimenters with wide latitude for both the sample enclosure and reference material choice and is thus widely applicable. The instrument is described and applied to the well-studied octadecyltrichlorosilane (OTS) film. The OTS spectra support the interpretation that variation in fabrication solvent water content and substrate preparation account for differences in OTS spectra reported in the literature.