Theoretical Procedure\nfor Precise Evaluation of Chemical\nEnhancement in Molecular Surface-Enhanced Raman Scattering

The enhancement of the molecular Raman signal in plasmon-assisted\nsurface-enhanced Raman scattering (SERS) results from electromagnetic\nand chemical mechanisms, the latter determined to a large extent by\nthe chemical interaction between the molecules and the hosting plasmonic\nnanoparticles. A precise quantification of the chemical mechanism\nin SERS based on quantum chemistry calculations is often challenging\ndue to the interplay between the chemical and electromagnetic effects.\nBased on an atomistic description of the SERS signal, which includes\nthe effect of strong field inhomogeneities, we introduce a comprehensive\napproach to evaluate the chemical enhancement in SERS, which conveniently\nremoves the electromagnetic contribution inherent to any quantum calculation\nof the Raman polarization. Our approach uses density functional theory\n(DFT) and time-dependent DFT to compute the total SERS signal, together\nwith the electromagnetic and chemical enhancement factors. We apply\nthis framework to study the chemical enhancement of biphenyl-4,4′-dithiol\nembedded between two gold clusters. Although we find that for small\nclusters the total SERS enhancement is mainly determined by the chemical\nmechanism, our procedure enables removal of the electromagnetic contribution\nand isolation of the contribution of the bare chemical effect. This\napproach can be applied to reproduce and understand Raman line activation\nand strength in practical and challenging SERS configurations such\nas in plasmonic nano- and pico-cavities.