Interfacial electric fields play an integral role in a variety of today’s most challenging problems throughout the areas of catalysis and device physics. Unfortunately, such fields are difficult to directly measure or to model theoretically and so they often remain less understood. We present here our recent experimental work that attempts to directly measure electric fields at a surface by combining the surface sensitivity of Sum Frequency Generation Spectroscopy (SFG) with the proven technique of Stark Shift Spectroscopy. In short, we measure the SFG spectrum of a self assembled monolayer of 4-Mercaptobenzonitrile covalently attached to gold and submersed in a variety of solvents. We observe an increasing red shift of the center frequency of the CN-stretch with increasing dielectric constant of the solvent. In order to interpret these results we have adapted the well-known Onsager model to the surface where the inversion symmetry of the bulk is broken and one side of the cavity is a conductor. This simple model, though somewhat primitive, adequately explains our observed CN-stretch frequencies as a function of solvent dielectric constant and in turn allows us to estimate the reaction fields of these solvents near the interface. Finally, we observe the signatures of specific interactions between the dielectric and the nitrile, such as hydrogen bonding and halogen bonding, that are not accounted for in the surface modified Onsager model. This new combination of two proven experimental techniques holds much promise for helping to elucidate electric fields near interfaces and how such fields affect chemical reactions at the surface.

Vibrational sum frequency generation Stark shift spectroscopy:  Measuring the electric field at a metal-dielectric interface

253rd National Meeting (2017)

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.