Probing interactions and reactions at complex interfaces – those consisting of soft, dynamic materials, small structures such as nanoparticles, or multicomponent mixtures – with molecular specificity is a challenge. The challenge is intensified when the interface is buried between two condensed phases. The only current techniques capable of probing these buried interfaces are the nonlinear spectroscopies: second harmonic generation (SHG) and its vibrational cousin, sum frequency generation (SFG). Nonlinearity brings surface specificity, but at a price: the signal from minor species either squares into the noise or gets swamped by interference with the signal from majority species. The solution to this dilemma has been known for some time: measure the complex (in the mathematical sense) amplitude rather than the intensity. The amplitude is linear in all components. This contribution will describe an interferometer – called a nonlinear interferometer – that eliminates artifacts due to Fourier transform and windowing, as well as building in methodology for active stabilization that are lacking in current methods. The nonlinear interferometer provides a method for measuring various standards commonly used against an absolute standard. It is argued that Z-cut quartz, +X oriented is the absolute standard of choice since the signal is insensitive to surface contaminants, is stable, and can be used from low frequency infrared through the ultraviolet. Application of nonlinear interferometry to liquid-air, thin film-air, nano particle films and high vapor pressure solutes will be discussed.

Thin films, aqueous-air, and nanoparticle interfaces

ACS Fall 2021

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.