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 Ag2-pyrazine complex, which have enabled us to find two excited singlets, namely CT0;1B1 and CT1;1A2, with properties quite similar to those of the pyrazine radical anion in its electronic B3u2 and Au2 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 S0;1A1 and both CT levels of the surface complex.