Mie resonances: Sensors for physical and chemical cluster interface properties

Abstract

Metal clusters exhibit unique optical properties due to the excitation of Mie plasmon resonances. It is well known since decades that measured resonances of clusters, surrounded by some adsorbate, or some solid or liquid embedding material (as e.g., in colloidal systems), are often not described quantitatively by Mie's theory. Only recently, these discrepancies were traced back to complex physical and chemical influences of the cluster-matrix interlayer onto the optical response. They prove often to be more important than cluster size effects.

These findings opened a new field of surface/interface research where deviations of measured Mie resonances from the predictions of Mie's theory are used as sensitive sensors for physical and chemical interface properties and processes in cluster-matter.

By combining optical spectroscopy experiments on free clusters in UHV and on the same clusters after embedding, this method was calibrated to separate, quantitatively, the cluster-matrix interface effects from other cluster effects like shape and structure effects, nonlocal dielectric effects and cluster size effects.

Among all metals, silver exhibits the most pronounced Mie resonances, so silver clusters were used as model systems and were embedded in a broad variety of solid and liquid embedding media, in course of the investigations reported in the present Progress Report.

A theoretical description of the obtained data, based upon static and dynamic charge transfer processes of the cluster electrons into/out of adsorbate states is, however, only at its beginning. It allows to ascribe the extremely short decay times of the resonances of the order of 1 to 10 fs to phase relaxation processes; the decay times are in good correspondence with results of direct femtosecond-experiments.