Understanding the Molecule-Surface Chemical Coupling in SERS Seth M. Morton and Lasse Jensen* The PennsylVania State UniVersity, Department of Chemistry, 104 Chemistry Building, UniVersity Park, PennsylVania 16802 Received November 22, 2008; E-mail: jensen@chem.psu.edu Abstract: The enhancement mechanism due to the molecule-surface chemical coupling in surface-enhanced Raman scattering (SERS) has been characterized using time-dependent density functional theory. This has been achieved with a systematical study of the chemical enhancement of meta- and para-substituted pyridines interacting with a small silver cluster (Ag 20 ). Changing the functional groups on pyridine enabled us to modulate the direct chemical interactions between the pyridine ring and the metal cluster. Surprisingly, we find that the enhancement does not increase as more charge is transferred from the pyridine ring to the cluster. Instead, we find that the magnitude of chemical enhancement is governed to a large extent by the energy difference between the highest occupied energy level (HOMO) of the metal and the lowest unoccupied energy level (LUMO) of the molecule. The enhancement scales roughly as (ω X /ω j e ) 4 , where ω j e is an average excitation energy between the HOMO of the metal and the LUMO of the molecule and ω X is the HOMO-LUMO gap of the free molecule. The trend was verified by considering substituted benzenethiols, small molecules, and silver clusters of varying sizes. The results imply that molecules that show significant stabilization of the HOMO-LUMO gaps (such as those that readily accept π-backbonding) would be likely to have strong chemical enhancement. The findings presented here provide the framework for designing new molecules which exhibit high chemical enhancements. However, it remains a challenge to accurately describe the magnitude of the Raman enhancements using electronic structure methods, especially density functional theory, because they often underestimate the energy gap. Introduction Raman spectroscopy is a powerful technique for determining structural information about a system, but the small cross section of Raman scattering typically results in low resolution and/or the need for high concentrations of analyte. 1 However, for analytes near rough metal surfaces, the signal can be enhanced by factors as large as 10 10 -10 15 due to the strong local field arising from the plasmon excitation combined with the direct chemical interactions between the molecule and the metal surface. 1-8 This is known as surface enhanced Raman scattering (SERS) and has potential to be exploited as a single-molecule spectroscopy. 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