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

TL;DR

This study investigates the origin of anomalous far-infrared absorption in small metal particles, revealing a crossover energy where absorption behavior shifts from quantum to classical, suggesting electron transitions between discrete energy levels as the cause.

Contribution

It proposes a new explanation for the anomalous far-IR absorption based on electron transitions below a crossover energy, linking quantum and classical absorption regimes.

Findings

Existence of a crossover energy Ec ~ 160 cm^-1 where absorption behavior changes.

Absorption above Ec is classical and size-independent.

Electron transitions between discrete energy levels cause the anomalous absorption.

Abstract

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.