Elucidation of charge-transfer SERS selection rules by considering the excited state properties and the role of electrode potentialElectronic supplementary information (ESI) available: Excited state energies and spectra, type of transitions, bond lengths, Cartesian gradient matrix, vibrational displacement and VEDA results, and calculated and fitted dimensionless displacement, as well as the Fermi level of the clusters. See DOI: 10.1039/c6cp07585a

• Main Authors: Mohammadpour, Mozhdeh, Khodabandeh, Mohammad Hassan, Visscher, Lucas, Jamshidi, Zahra
• Format: Article
• Language: English
• ISSN: 1463-9076; 1463-9084
• DOI: 10.1039/c6cp07585a

The goal of this study is to shed light on the charge-transfer (CT) mechanism of surface-enhanced Raman scattering (SERS) by considering the properties of CT excited states. The calculations have been done by means of an excited-state gradient approximation for a pyridine molecule interacting with a silver cluster, and provided a satisfactory improvement in comparison to previous work. The effect of electrode potential on the SERS-CT spectra has been modelled theoretically by applying an external electric field for selected CT transitions and the enhancement of the ν 6a and ν 9a modes and a decline in the intensity of the ν 8a mode under a negative electric field (which is directed toward the cluster) have been observed. These results match well with the experimental studies and also explain the effect of electrode potentials on the patterns of spectra, as experimental evidence of the CT mechanism. Finally, this study demonstrated that the excited state vector gradient can be used as a distinguishing factor to explain the SERS selection rules. The charge-transfer mechanism contributing to SERS is investigated by examining the properties of the excited CT-state and contributions of the electrode potential.