Adsorption of molecules on the surface of small metal particles studied by optical spectroscopy

Abstract

The influence of adsorbate layers on surface plasmon excitation of small supported metal particles with mean sizes ranging from R=20 to 40 nm has been investigated. For this purpose, sodium and potassium clusters adsorbed on dielectric substrates served as model systems. Their optical transmission spectra were first recorded under ultra-high vacuum conditions directly after preparing the particles and, subsequently, after dosage of gases such as O 2, N 2O, CO 2, H 2 and N 2. The optical spectra are dominated by two maxima that originate from the excitation of surface plasmon resonances in the direction of the long and short axis of the oblate particles. Changes are found upon adsorption of as little as 0.04 monolayers on the cluster surface. The results of a series of measurements together with a theoretical treatment using the quasi-static approximation indicate that the variations of the spectra allow one to characterize the strength of the surface chemical bond. In addition, diffusion of the atoms into the bulk of the particles can be followed. Particularly interesting is the observation that the clusters can experience a change in shape if gases such as O 2 or CO 2 react with their surface.