Particle-like absorption spectra in small transition metal clusters

Abstract

Research at Argonne has found that small clusters of certain transition metal atoms display optical absorption spectra that are fundamentally different from those typically observed for other small molecules. A new laser technique known as photodissociation spectroscopy was used to record the absorption spectra of nickel clusters containing three to seven atoms and of niobium clusters containing between seven and twenty atoms. Instead of displaying spectra with distinct and separate absorption {open_quotes}bands{close_quotes} as is usually seen for small molecules, these metal clusters possess absorption spectra that are continuous and smooth throughout the visible and near ultraviolet regions, indicating that they absorb light at every wavelength. This behavior is in fact more like that displayed by much larger metal particles containing hundreds or thousands of atoms. This surprising result may indicate that the electronic structures of these small clusters are much closer to those of a corresponding macroscopic piece of nickel or niobium metal than was previously thought. These studies are aimed at understanding the electronic structure of the active metal cluster sites in catalysts, and may in addition provide valuable insights into the design and properties of new cluster assembled materials.