Quantifying Electrostatic Contributions to the Nonresonant Chemical SERS Enhancement

Molecules near metal surfaces experience an enhancement of Raman scattering signals. This is termed as surface-enhanced Raman scattering (SERS) and has two enhancement mechanisms: the electromagnetic mechanism and the chemical mechanism. In this work, we present a systematic study of the nonresonant chemical mechanism (CHEM) for CO, N₂ , CS, and pyridine interacting with Cu_x , Ag_x , and Au_x clusters ranging in size from x = 4–80 atoms. We show that CHEM scales as 1 + A_k^m |q^inter |⁴ , where |q^inter | is the induced charge flow between the molecules and the metal clusters, and A_k^m is a molecular- and vibration-specific scaling parameter. Surprisingly, we find that the scaling parameter is smallest for the shortest adsorption bond lengths and that for different modes, it scales with the electron–phonon coupling. In addition, we show that CHEM can be decomposed into an electrostatic contribution arising from polarizing the metal cluster and a contribution due to electron density reorganization from the binding of the molecule to the cluster. This electrostatic contribution can be quantified using an atomistic electrodynamics/quantum mechanical model and is shown to account for about 1 order of magnitude of enhancement. The remaining enhancement due to electron density reorganization is likewise found to contribute about 1 order of magnitude. For molecules with shorter adsorption bond lengths, we find that the electrostatic contribution increases, while the electron density reorganization contribution decreases, explaining why stronger bonding does not lead to greater enhancement. Finally, this decomposition enables the identification of vibrational modes that are more likely to be sensitive to the chemical mechanism. In summary, this work provides new insights into the chemical enhancement of SERS by decomposing CHEM into an electrostatic contribution and a contribution due to electron density reorganization between the molecule and the metal cluster.