A look at the origin and magnitude of the chemical contribution to the enhancement mechanism of surface-enhanced Raman spectroscopy (SERS): Theory and experiment

Normal and surface-enhanced Raman spectra for a set of substituted benzenethiols were measured experimentally and calculated from static polarizability derivatives determined with time-dependent density functional theory (TDDFT). Both silver and gold cluster-thiolate complexes were studied to investigate how the chemical enhancement varies with substituent. The experimental relative peak intensities and positions are well-matched by their theoretical counterparts. The static chemical enhancement of the ring stretching modes near 1600 cm^-1 is determined experimentally and computationally for each derivative, and it is found that the experimental enhancement varies by a factor of 10 as a result of chemical substitution, with stronger electron donating groups on the benzene unit leading to higher enhancements. The calculated trends with substitution match experiment well, suggesting that TDDFT is describing the chemical effect qualitatively, if not quantitatively, in the static (low-frequency) limit. A two-state model is developed, providing qualitative insight into the results in terms of the variation of ligand-to-metal charge-transfer excitation energy with substitution.