Stark effect or coverage dependence? Disentangling the EC-SEIRAS vibrational shift of sulfate on Au(111)

Infrared spectroscopy is a widely employed analytical tool in (electrochemical) surface science as the spectra contain a wealth of information about the interaction of interfacial adsorbates with their environment. Separating and quantifying individual contributions, for example, of co-adsorbates, the substrate or electric field effects, on the overall spectral response, however, is often non-trivial as the various interactions manifest themselves in similar spectral behavior. Here, we present an experimental approach to differentiate between and quantify potential-induced coverage dependence and field-related Stark effects observed in a sulfate band shift of 93.5 ± 1.5 cm^-1/V in electrochemical infrared spectra of the showcase sulfate/Au(111) interface. In combination with a simple linear model equation used to describe the potential-induced peak shift of the sulfate stretch vibration, we determine the coverage dependence contribution to be 15.6 ± 1.2 cm^-1/θ_SO and the Stark effect to amount to 75.6 ± 2.7 cm^-1/V. Our work provides a novel route to gain fundamental insight into interfacial adsorbate interactions in electrochemical surface science.