In this project supported by the Chemical Structure, Mechanisms and Dynamics Program, Professor John Fourkas of the University of Maryland and his research group will develop a technique called parametrically amplified vibrational sum frequency generation (PAVSFG) for the study of the structure and dynamics of surfaces and interfaces. PAVSFG is an advanced version of conventional sum frequency generation spectroscopy, which is based on a second order nonlinear optical (NLO) process in which two low energy photons combine at a surface or interface to produce a high energy photon.
Since second order NLO processes are very weak in intensity, the parametric amplification process developed in the Fourkas lab will have positive consequences for a wide range of surface and interfacial science investigations, including biological membranes and catalyst surfaces. This project will also be the vehicle for training students and post-doctoral researchers in cutting-edge optical physics and physical chemistry.
Liquid interfaces play a crucial role in a wide variety of technologies, including lubrication, oil recovery, and remediation of groundwater pollution. Thus, understanding how the properties of liquids change at an interface is of crucial importance. However, typically the number of liquid molecules in contact with a surface is a tiny fraction of the number of molecules in the rest of the liquid, which makes it difficult to study these interfacial molecules. The purpose of this project was to enhance a well-established technique for probing interfacial molecules, vibrational sum-frequency-generation spectroscopy, by incorporating a novel scheme for optical amplification of the signal. This new method should allow spectra to be obtained in a single laser shot, providing rapid data collection and the ability to study samples that may be destroyed after a few (or even one) shots of the laser. We have implemented optical amplification, and along the way have made other improvements to this technique that should be of broad benefit to other researchers who use this technique and to the problems and applications that they study. In addition, we have developed a new method of selecting individual laser pulses that will have broad applicability across the field of laser science and spectroscopy. From an experimental standpoint, we have taken advantage of the increased sensitivity of our instrument to study the behavior of liquid mixtures near a silica surface. We have discovered a unique microscopic structure in a technologically important solvent mixture, acetonitrile/water. The microscopic information provided by our studies should facilitate improvements in the ability to separate mixtures of molecules, which is a key step in chemical purifications. As such, these results have broad implications in areas including pharmaceutical research.
