Far-Infrared Absorption in Ultrafine Al Particles

The first far-infrared measurements on metallic particles with diameters < 10 nm are reported. The absorptivity is well approximated by ${\alpha }_{\exp }\approx C_{\exp }{\tilde{\nu }}^2$, where $3<\mathrm{}\tilde{\nu }<150\mathrm{}$ ${\mathrm{cm}}^{-1\mathrm{}}$. 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, ${\alpha }_{\mathrm{GE}}=C_{\mathrm{GE}}{\tilde{\nu }}^2$ irrespective of level statistics. However, $C_{\exp }\gg C_{\mathrm{GE}}$ 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.