CHARACTERIZATION OF LARGE SUPPORTED METAL-CLUSTERS BY OPTICAL SPECTROSCOPY

Small sodium and silver particles were generated on dielectric substra tes like LiF, quartz and sapphire under ultrahigh vacuum conditions. T he optical transmission spectra of the clusters were measured as a fun ction of cluster size and shape, for low and high substrate temperatur es as well as for s- and p- polarization of the incident light. Excita tion of dipolar surface plasmon oscillations in the directions normal and parallel to the substrate surface could be identified. Furthermore , optical spectra for Na and Ag clusters were calculated with the clas sical Mie theory. The measured spectra vary strongly if the experiment al conditions are changed and can be exploited, for example, to charac terize the particles with regard to their size and shape. In particula r, the axial ratio of the spheroidal clusters could be determined. Its value is considerably different for the two investigated metals and d epends on the substrate material. Furthermore, the temperature of the substrate has a pronounced influence on the shape of the particles. At low temperature of T = 100 K two-dimensional island growth is predomi nant. The particles extend only little in the direction perpendicular to the surface and coalesce readily at small coverage of metal atoms. In contrast, the clusters are truly three-dimensional at T = 300 K. At this stage, sodium particles still exhibit a rather small axial ratio whereas silver clusters appear almost spherical. Thus, measurements o f the optical spectra permit direct in situ monitoring of cluster grow th during the nucleation of adsorbed atoms and of temperature induced shape variations. In addition to investigations of the shape of the pa rticles, the quadrupolar surface plasmon mode was observed for Ag clus ters.