Possible origin of the anomalous far-infrared absorption by small metal particles

We carried out the far-infrared transmission measurements on the ligand stabilized Au55 particles dispersed in CsI. We found that the far-infrared absorption coefficient recovers the ω2-dependence (the same behavior as the classical electric dipole-like absorption) for frequencies above the crossover energy Ec=ℏ2/2mea2Ec=ℏ2/2mea2∼160 cm−1 (a=particle radius) while the absorption coefficient below Ec, which arises from the transitions of the electrons between the discrete energy levels, saturates to a value ∼4 cm−1 at ω∼Ec. Since Ec of the small metal particles (a>20 Å) studied in the past falls well below the lowest frequency typically accessible for the far-infrared spectroscopy (ω∼20 cm−1), we conclude that all the previously measured far-infrared absorption must have been an extension of the absorption of the quantum regime set at Ec, making the absorption measured in the typical far-IR window (20 cm−1<ω<120 cm−1) anomalously large when compared with the classical electric dipole absorption by the same size particles.

► We found the crossover energy (Ec) above which the classical behavior sets in.
► The far-IR absorption below Ec resembles that of a semiconductor of an energy gap Δ∼10 cm−1.
► The far-IR absorption appears as an extension of the absorption at v=Ec.