Role of the electrode potential in the charge-transfer mechanism of surface-enhanced Raman scattering

A general model concerning the role of the electrode potential on the charge-transfer enhancement mechanism of SERS is proposed, justifying the existence of a parameter P which reduces the effectiveness of the electrode potential in order to shift the excited charge-transfer level. This explains the observed deviation of 0 from the unity, and allows for relating the intensity/electrode-potential (SERS-CT) profiles of similar adsorbates with the respective electron affinities. The SERS of pyridine, pyrazine and methylpyrazines have been recorded on silver, being found that the electrode potential in the maximum of the SERS-CT profiles shifts toward more negative values as the number of methylsubstituents attached to the pyrazine ring increases. This result has been explained on the basis of the relative stability of the radical anions given that a correlation between those shifts and the calculated ab initio vertical transition energies between the neutral molecules and their respective radical anions can be established. This confirms the participation of a resonant charge-transfer mechanism in the SERS as well as the usefulness of this technique to gain insight into the electronic properties of the doublet states of the here studied molecules.