Doubly-resonant sum frequency generation spectroscopy as a sensitive probe of structure and orientation at interfaces

COLL 258

Neil A. Anderson1, Kimberly A. Briggman1, Lee J. Richter2, and John C. Stephenson1. (1) Optical Technology Division, National Institute of Standards and Technology, 100 Bureau Drive, MS8443, Gaithersburg, MD 20899, (2) Surface and Microanalysis Science Division, National Institute of Standards and Technology, 100 Bureau Drive, Gaithersburg, MD 20899
Vibrationally-resonant sum frequency generation spectroscopy (VR-SFG) has developed into an important tool for exploring molecular structure and orientation at interfaces. However, VR-SFG lacks sufficient sensitivity to probe extremely low interfacial molecular concentrations for applications in biological systems, e.g. physiological concentrations of proteins in model membranes and low-density DNA and protein chips. Analogous to resonant Raman spectroscopy, doubly-resonant SFG offers improved sensitivity and selectivity by enhancing the signal from vibrational modes that are coupled with a resonant electronic transition. This work presents ultraviolet doubly-resonant SFG studies of self-assembled monolayers of 4,4'-di(phenylene-ethynylene)benzenethiol (OPE), a molecular electronic switch molecule, and thiolated DNA base analogs on gold. Orientational and structural analyses of these smaller molecules help to lay the groundwork for application of this novel technique to more complex systems.