CHARGE-TRANSFER RESONANCE RAMAN PROCESS IN SURFACE-ENHANCED RAMAN-SCATTERING FROM P-AMINOTHIOPHENOL ADSORBED ON SILVER - HERZBERG-TELLER CONTRIBUTION

The surface-enhanced Raman scattering (SERS) of p-aminothiophenol adso rbed on silver measured with visible excitations is compared with the normal Raman and ultraviolet resonance Raman (UVRR) scattering of the free molecule. The SERS is dominated by four b(2) and one a(1) symmetr y species of benzene ring vibrations (under an assumption of C-2v symm etry), whereas the normal Raman scattering at visible excitations only by a(1) species. The b(2) modes corresponding to the strong SERS band s are observed for the free molecule only when the electronic transiti on from the ground state to the lowest unoccupied molecular orbital (L UMO, pi b(2), lambda(max) similar to 300 nm) is excited. In electroch emical environments, the b(2) SERS bands show resonance-shaped intensi ty profiles as a function of applied potential. The intensity-potentia l profiles shift to positive potentials as the excitation energy incre ases. This is well interpreted by a resonance Raman-like process assoc iated with the photon-induced charge transfer from the metal to an aff inity level of the adsorbed molecule. The affinity level is assigned t o LUMO on the basis of the UVRR experiments and a symmetry considerati on. The b(2) modes gain their intensities via a Herzberg-Teller (vibro nic) term in both SERS and UVRR scattering. Due to the difference in t he electronic levels vibronically coupled, however, the enhancement pa tterns are greatly different.