Interpreting the chemical mechanism in SERS using a Raman bond model

We present a first-principles model that partitions Raman intensities to atomic and bond contributions. This framework allows us to interpret the chemical mechanism in surface-enhanced Raman scattering (SERS) as interatom charge flow modulations, which we define as Raman bonds. Hirshfeld partitioning and charge density localization are applied to express polarizability derivatives as charge flow modulations. Model systems consisting of pyridines, thiols, and carbenes interacting with metal clusters are studied using time-dependent density functional theory. We demonstrate that the mode-specific enhancements can be explained as Raman bonds conjugated across the molecule-metal interface. We also illustrate that the changes in Raman intensities induced by electric fields or chemical substitutions can generally be interpreted as changes of charge flows. The model is shown to work consistently for different types of molecule-metal bonds. Furthermore, our work shows that increasing the Raman bond conjugation across the interface leads to stronger chemical enhancements. The Raman bond model developed in this work provides a quantitative and intuitive interpretation of the chemical mechanism in SERS.