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Possible Origin of the Anomalous Far-infrared Absorption by Small Metal Particles

We have carried out the far-infrared transmission measurements on the ligand stabilized Au55 particles dispersed in CsI. We have found that there exists the crossover energy Ec ~ 160 cm -1 (~ 19 meV) beyond which the absorption coefficient recovers the classical electric dipole-like absorption and, hence, the absorption for frequencies higher than Ec is insensitive to the particle size. We propose that the physical origin of the enigmatic anomalous far-IR absorption by small metal particles lies in the absorption arising from the transitions the electrons between the discrete energy levels below Ec on which the particle size independent, classical electric dipole absorption develops for frequencies higher than Ec.