The role of charge-transfer states of the metal-adsorbate complex in surface-enhanced Raman scattering

Surfaced-enhanced Ramon scattering (SERS) spectra of pyrazine are analyzed on the basis of the properties of the electronic states of the metal-adsorbate surface complex. Ab initio CIS calculations have been carried out for the ${\mathrm{Ag}}_2$ -pyrazine complex, which have enabled us to find two excited singlets, namely ${\mathrm{CT}}_0{;}^1{B}_1$ and ${\mathrm{CT}}_1{;}^1{A}_2,$ with properties quite similar to those of the pyrazine radical anion in its electronic ${}^2{B}_{\text{3u}}$ and ${}^2{A}_u$ states, respectively, and with energies falling in the range of the exciting photons usually employed in Raman spectroscopy. SERS spectra of pyrazine are compatible with a resonance Raman enhancement mechanism involving electronic transitions between the ground state $S_0{;}^1{A}_1$ and both CT levels of the surface complex.