Detecting photoinduced electron transfer processes in the SERS spectrum of salicylate anion adsorbed on silver nanoparticles

The surface-enhanced Raman scattering (SERS) of salicylic acid (S) adsorbed on a silver sol in H2O and D2O has been investigated. At pH 5 or greater, the adsorbed species is the salicylate anion (2-hydroxybenzoate anion) (S-), which links to the metal nanoparticle (Ag-n) through the carboxylate group (S--Ag-n). We demonstrate that the selective enhancement of the bands is due mainly to a resonant electron or charge transfer process (ET or CT) from the metallic nanoparticle to the adsorbate, yielding the transient formation of the respective radical dianion (S.(2-)-Ag-n(+)). It is found that the enhanced bands, and especially mode 8a;v(ring), are related to the differences between the equilibrium structures of the adsorbate in its ground (S-) and CT-excited states (S.(2-)). (c) 2006 Wiley Periodicals, Inc.