Unusual response of the chemical contribution to the plasmonic excited near field in enhanced Raman spectra of molecules on metal substrates

Surface-enhanced vibrational spectroscopy is a powerful tool widely used in scientific research and industry for label-free and sensitive identification of molecular species. Two main enhancement mechanisms, namely, electromagnetic enhancement and chemical enhancement, have been accepted to be collectively responsible for striking modifications in the enhanced spectra of molecules absorbed on metal plasmonic substrates. However, the two effects and their respective contribution in various metal- or dielectric-molecule systems remain imperfectly understood. Here we exemplify a hybrid molecule-metal system and decouple the two contributions to surface-enhanced Raman-scattering (SERS) intensity. Our findings demonstrate the nanoscale near-field enhancement can be estimated with only the measured far-field SERS intensity. Notably, the chemical effect on the SERS signal exhibits an essentially different response against the enhanced near field between molecular vibrational lines. Hence, this work may be helpful in better understanding the origin of the anomalously changed Raman patterns when adsorbates upon metal surfaces, but also offers heuristic insights into the underlying interactions between various enhancement effects in surface-enhanced vibrational spectroscopy.