Complex surface containing multiple species are common in biology, the environment, and technological applications. Probing these, particularly when the interface is buried between two condensed phases, presents many challenges. The only current method capable of probing such interfaces with molecular specificity is the vibrational spectroscopy, sum frequency generation (SFG). SFG attains surface sensitivity due to its nonlinear nature, but that nonlinearity carries a price: deconvoluting overlapping signals is difficult, usually leading to nonunique separation. One consequence is that it is rarely possible to measure surface concentration: a fundamental quantity for tracking mechanisms. This problem as well as its solution has long been recognized by SFG practitioners: one merely has to measure the complex (in the mathematical sense) response. Thus, several methods have been devised to determine the complex signal. None produce an absolute measurement of the complex signal and all have limited accuracy. This contribution reports a nonlinear interferometer, that produces interferometeric precision enabling detection of even low concentration interfacial species. The nonlinear interferometer has been demonstrated in both scanning and broadband SFG systems. Thin films and other interfaces will be discussed. Specifically, we show that the conformation of molecules in a hydrophobic interface are significantly altered by water in the film.

Nonlinear interferometric detection of complex surfaces

2019 Fall National Meeting

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