The anomalous infra-red absorption of alkali metals and collective oscillations in small metal particles

It is shown that the resonance like absorption peaks, which were discovered by Mayer for bulk samples of Na and K, and which could not be explained in terms of the bulk properties of the metal, are actually due to a surface roughness effect. The ellipsometric method leads, indeed, to a systematic error when the metal surface is covered by a dense distribution of small metal particles. The apparent optical constants depend on the optical constants of the bulk metal and on the effective optical constants of the thin granular film. Resonance absorption is particularly strong, when it occurs in the spectral region where the absorption of the bulk metal is very small, as it happened to be for these alkali metals.