Abstract

We have identified the long debated origin of the short-range component of surface-enhanced Raman scattering (SERS) as Ag₄⁺ clusters which are stabilized by co-adsorbed molecules or by low substrate temperature on rough Ag surfaces. The assignment has been supported by vibrational mode calculations which include the effects of adsorption of the cluster, image interaction with the metal, and co-adsorbed anions in the electrochemical environment. The observed modes at 73, 110, and 161 cm^-1 correspond to those calculated at 75, 100 and 118, and 170 cm^-1. The adsorbed clusters act as Lewis acid sites onto which other molecules such as pyridine are bonded thus leading to SERS beyond the usual electromagnetic factor which is due to macroscopic roughness. The mechanism of the added enhancement is through charge transfer excitation between the adsorbed molecules and the clusters and through HOMO—LUMO transitions within the cluster.

© 1986 Optical Society of America