UC San Diego

Heterodyne 2D sum frequency generation spectroscopy is used to study a model CO2 reduction catalyst, Re(diCN-bpy) (CO)3Cl, as a monolayer on a gold surface. We show that short-range interactions with the surface can cause substantial line-shape differences between vibrational bands from the same molecules. We explain this interaction as the result of couplings between CO vibrational modes of the catalyst molecules and the image dipoles on gold surface, which are sensitive to the relative distance between the molecule and the surface. Thus, by analysis of HD 2D SFG line-shape differences and polarization dependences of IR spectra, we can unambiguously determine the ensemble-averaged orientation of the molecules on the surface. The high sensitivity of HD 2D SFG spectra to short-range interactions can be applied to many other adsorbate-substrate interactions and therefore can serve as a unique tool to determine adsorbate orientations on surfaces.