Influence of molecular adsorbate layers on the optical spectra of small metal particles

The influence of molecular adsorbate layers on surface plasmon excitation of small supported metal particles has been investigated under ultrahigh vacuum conditions. For this purpose sodium clusters adsorbed on quartz substrates served as a model system. Their optical transmission spectra were recorded immediately after preparation and, subsequently, after adsorption of O-2, N2O, CO2 or N-2 on the particle surface. Depending on the molecules to which the clusters are exposed, different changes of the surface plasmon resonance in the spectra can be identified. The most essential experimental results are as follows. First, changes of the optical spectra are found if the clusters are covered by less than a molecular monolayer. Secondly, the observed variations of the spectra allow to distinguish between physisorption and chemisorption, i.e. characterize the strength of the surface chemical bond. Third, in sharp contrast to the usual shift of the plasmon frequency to longer wavelengths by change of the dielectric surrounding, a blue shift of the resonance has been observed upon dosage of CO2. Finally, diffusion of adsorbate molecules into the bulk of the particles can be identified.