The first far-infrared measurements on metallic particles with diameters 10 nm are reported. The absorptivity is well approximated by αₑₓₚ≈Cₑₓₚ ̃ \~ν², where 3 ̃ \~ν150 cm^-1. The results are compared with the Gor'kov-Eliashberg (GE) theory of particles with quantized electronic levels as well as with the classical Drude theory. When a size distribution is included, αGE=CGE ̃ \~ν² irrespective of level statistics. However, CₑₓₚCGE and the observed dependence on the diameter is directly contrary to predictions of the GE theory. Neither can the data be reconciled with the classical theory.
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Granqvist et al. (1976) studied this question.