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 Ag-2-pyrazine complex, which have enabled us to find two excited singlets, namely CT0;B-1(1) and CT1;(1)A(2), with properties quite similar to those of the pyrazine radical anion in its electronic B-2(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. (C) 2002 American Institute of Physics.