Surface-enhanced linear and nonlinear vibrational spectroscopy from first-principles

Lasse Jensen from the Pennsylvania State University is supported by an award from the Chemical Theory, Models and Computational Methods program to develop new theoretical tools for analyzing and predicting spectroscopy of molecules near nanoparticles or nanostructured interfaces with surface plasmons. These may be, for example, particles of gold that are several nanometers (billionths of a meter) in size. Surface plasmons are collective electronic motions that strongly absorb and scatter incoming light of particular color ranges. This project aims at developing and distributing computational tools to analyze and understand exactly how molecules and light interact in such circumstances. Particular attention is paid to the unique situation that light scattering is triggered by two incoming photons of light instead of the usual circumstance of a single incoming photon. The resulting computational tools will be important for many applications in catalysis of chemical reactions, conversion of light to energy, and biological imaging. They will be disseminated to a broad scientific community through incorporation into available chemical compuational software packages. An interactive Nano-Optics Simulator visualization module will be integrated into summer research experiences for high school students and made available to a larger audience through the nanoHub, a central web portal for nanoscience. Undergraduate students are trained in cutting-edge science by participating in this research. Graduate students gain research expertise, mentoring experiences, and collaborative skills through their participation in both research and educational activities. Understanding multi-photon properties at nano-interfaces is of great importance for applications in all-optical switching, energy-up conversion, heterogeneous catalysis and biological imaging. Surface-enhanced Raman scattering (SERS) and its nonlinear analogue surface-enhanced hyper-Raman scattering (SEHRS) can be used to provide detailed information about molecular excited state properties down to the single-molecule level. This research addresses the main challenges in understanding SERS and SEHRS by developing accurate and efficient methods to directly simulate the spectra from first principles. These methods are being used to 1) understand the enhancement mechanisms, 2) establish limits on the possible enhancements, and 3) design plasmonic substrates that are targeted for SEHRS applications. Dissemination is planned through incorporation into sofware packages such as ADF and NWChem. Ultimately, this research is aimed at a detailed molecular level understanding of nonlinear optical properties in novel materials.