Modulated surface-plasmon resonance for adsorption studies

Surface-plasmon resonance (SPR) on high-reflectivity metals is very sensitive to changes in the optical properties of the surface region, in particular, to the effects of adsorbed molecules. We excite the SPR on noble-metal films using the Kretschmann configuration and detect the SPR via the surface-roughness-scattered light. By modulating the angle of incidence of the exciting laser beam and using the electronically differentiated signals to track and monitor the resonance, we are able to measure, as a function of time, the complex dielectric constant of the film and the strength of the surface-roughness scattering. We use this technique to study the chemisorption of ${\mathrm{O}}_2$ on Cu and Ag where we are able to detect the presence of a fractional monolayer of physisorbed ${\mathrm{O}}_2$ on the Ag film after the chemisorption is essentially complete. Data on the optical constants obtained from the metal-vacuum interface of in situ grown Ag, Cu, and Au films are presented. We also present data on the strong physical adsorption of 1,2-dichloroethane on room-temperature Ag films. At elevated temperatures, the dichloroethane attacks and roughens the surface. The increased roughness allows us to establish an upper limit of 1% for the contribution of the roughness scattering to the width of the SPR on the clean Ag film.