CLASSICAL AND CHEMICAL EFFECTS OF SERS FROM 2,2'-5,2'' TERTHIOPHENE ADSORBED ON AG-SOLS

Surface-enhanced Raman scattering (SERS) has been observed from 2,2':5,2" terthiophene (3T) adsorbed on Ag-sols. Experimental results suggest that the absorbed 3T molecules are anti but twisted and that the twist angle is smaller in the surface-adsorbed state than in solution. The first-layer effect, i.e., maximum enhancement when a monomolecular layer of 3T is adsorbed on silver particles, has been observed for the first time in such SERS active surfaces. A charge-transfer model has been shown to contribute to this effect. On the other hand, the Raman excitation frequency dependence of the enhancement suggests a significant classical electromagnetic contribution to SERS in sol.