Molecular-level measurements on high vapor pressure, soft or buried interfaces have long been recognized as challenging. Hence, the first report of a sum frequency generation (SFG) vibrational spectrum from methanol by Shen and coworkers opened a new vista on surfaces important to biological systems, atmospheric processes, corrosion, colloids and much more. However, it was quickly realized that the inherently complex nature of the SFG signal made interpretation not only challenging, but also made separating components of a mixture nonunique. For example, determining impact on the orientation or interactions between molecules of a major constituent from a smaller concentration additive requires measurement of the complex amplitude response, not simply the intensity. In addition, signal from the smaller concentration additive is generally buried in the noise due to the quadratic concentration dependence. This contribution reports an interferometric method for direct measurement of the absolute complex spectrum using a recently invented nonlinear interferometer. The instrument is a compound interferometer consisting of an embedded linear Mach Zehnder interferometer that provides both active stabilization, accuracy, and reproducibility. The interfering nonlinear signals are generated within the interferometer – hence the name nonlinear interferometer. We will report application to several surfaces as well as the specific impact of water on the configuration of self-assembled silane films.

Absolute, complex spectral measurement: Turning SFG (sum frequency generation) into an analytical technique

255th National Meeting (2018)

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