Collaborative Research: Using Nonlinear Light Scattering to Unravel the Chemical Effects in Surface Enhanced Spectroscopy

With support from the Chemical Structure and Dynamics (CSD) program in the Division of Chemistry, Professors Jon Camden at the University of Notre Dame and Lasse Jensen at Pennsylvania State University are combining sophisticated experimental and computational approaches to disentangle the relative contributions of electromagnetic and chemical enhancements in surface-enhanced spectroscopy. While it has been known for half a century that chemical effects play an important role in surface enhanced spectroscopies, a comprehensive understanding of these contributions remains elusive. Furthermore, employing current theoretical methods to predict the magnitude of chemical enhancements can be in error by several orders of magnitude, limiting their utility. Therefore, Professor Camden, Jensen, and their students will employ a non-traditional approach to understanding the chemical enhancement mechanism by combining experimental measurements using nonlinear spectroscopy with newly developed theoretical methods for calculating the nonlinear response properties. Their discoveries could advance the use of surface-enhanced spectroscopy by enabling high-quality predictions of the chemical enhancements and the rational design of molecular systems that maximize the spectroscopic response of molecules at surfaces. This work will additionally support a STEM teacher residency program and tools for visualizing molecular vibrations for the undergraduate chemistry curriculum, which will enable the proposed research to foster the next generation of STEM students. Specifically, this proposal addresses three outstanding fundamental scientific questions and challenges related to the chemical mechanism of surface enhanced spectroscopy. First, a comparison of surface-enhanced Raman scattering (SERS) and surface-enhanced hyper-Raman scattering (SEHRS) spectra of non-resonant probe molecules will be undertaken to address how static chemical effects can modify the overall enhancement factors. Second, a wavelength-scanned measurement of SEHRS spectra for resonant probe molecules will address how resonant chemical effects can modify the overall enhancement factors. Third, the first experimental measurements and theoretical calculations of non-degenerate SEHRS will be made to establish a benchmark and further characterize resonance effects in the enhancement mechanism. The experimental measurements will be complimented by the development of new computational approaches to model and interpret SEHRS. The combined experimental and theoretical studies will provide detailed insights into the chemical mechanism and serve as a comprehensive benchmark of the theoretical models. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.