Far-infrared absorption by small particles

Detailed far-infrared (FIR) and mid-infrared (MIR) studies of aluminum small particles have been carried out. MIR absorption studies of clustered and non-clustered samples suggest that the anomalous FIR absorption does not arise from the clustering together of the individual metallic particles. The MIR absorption, which is too small to be the continuation of the anomalous FIR absorption, could be fitted to the classical theory by using an unexpectedly short electronic mean free path. Evidence is presented for the particles having a non-Drude dielectric function, suggesting that the anomalous FIR absorption is due to the fact that the small particles are not metals.