Quantifying the enhancement mechanisms of surface-enhanced Raman scattering using a Raman bond model

In this work, a Raman bond model that partitions the Raman intensity to interatomic charge flow modulations or Raman bonds is extended from the static limit to frequency dependent cases. This model is based on damped response theory and, thus, enables a consistent treatment of off-resonance and resonance cases. Model systems consisting of pyridines and silver clusters are studied using time dependent density functional theory to understand the enhancement mechanisms of surface-enhanced Raman scattering (SERS). The Raman bonds in the molecule, the inter-fragment bond, and the cluster are mapped to the enhancement contributions of the molecular resonance mechanism, the charge transfer mechanism, and the electromagnetic mechanism. The mapping quantifies the interference among the coupled mechanisms and interprets the electromagnetic mechanism as charge flow modulations in the metal. The dependence of the enhancement on the incident frequency, the molecule-metal bonding, and the applied electric field is interpreted and quantified. The Raman bond framework offers an intuitive and quantitative interpretation of SERS mechanisms.