Optical response of small niobium clusters

The optical absorption spectra of small, isolated niobium clusters from seven to twenty atoms have been measured from 334 to 614 nm via photodepletion of niobium cluster-argon van der Waals complexes, ${\mathrm{Nb}}_{\mathit{n}}$ ${\mathrm{Ar}}_{\mathit{m}}$, in a moleculear beam. The absorption cross sections of all clusters in this size range increase with decreasing wavelength, as predicted by classical electrodynamic theory applied to small niobium spheres, however, the magnitudes of the cross sections are several times larger than predicted.