Plasmon resonances observed in light scattered by large alkali clusters

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Abstract. We studied the possibility of exciting plasmons bylight in arbitrarily large Li, Na, K, Rb and Cs clusters asa function of cluster size. We used the results of the Mietheory to describe the dependency of the total scattering (ex-tinction) cross-sections on the cluster radius. We also appliedthis theory to describe the intensity of the scattered light in di-rectionsorthogonalto the directionof the incident light beam.In the experiment we measured the intensity of light scatteredby growing sodium clusters in directions orthogonalto the di-rection of the incident light beam. We found that the maximaof the measured intensities arose from secondary fields due tothe dipole plasmon and the quadrupole plasmon resonances.PACS: 36.40.+d; 78.20.-eThe possibility of exciting and observing collective oscilla-tions in spherical metal clusters is a subject of growing in-terest [1–11]. In contrast to bulk metal with a flat surface,the curved surface of a spherical metal cluster enables directoptical excitation of collective motions of free electrons. Ex-tensive study of the scattering of light by a sphere of any sizewas made by Mie using classical electrodynamics. The Mietheory only deals with the problem of the continuity of thetangential component of the divergence-free electromagnet-ic field and does not deal with the phenomenon of collectiveelectron oscillations. Nevertheless, peaks in the absorptioncross-sections are usually interpreted as a manifestation ofresonances due to excitation of collective oscillations (plas-mons) (see e.g. [1,2,12,13]).In this paper we focus our attention on the optical proper-ties of metal clusters as a function of their size for differentalkali metals. We do not make any assumption concerningthe size of the cluster in comparison to the wavelength of thelight. We analyze the maxima in the scattering cross-sectionefficiencies for these metal spheres by combining the formal-ism of the classical Mie theory and the concept of plasmon-polariton oscillations. We treat the metal sphere filled withfree conduction-electrons as a cavity with size-dependenteigenfrequencies,l D1