The optical spectra of small isolated sodium clusters adsorbed on a boron nitride surface have been measured from 3000 to 7500 Å over a range of average cluster size 〈N〉=10 to 300 atoms by change in reflectivity spectroscopy. The spectra are dominated by the dipole resonance of the surface collective mode of the sodium valence electrons, and the resonance frequency shifts from 2.4 eV to near the bulk value of 3.4 eV as cluster size increases. A quantitative comparison of experimental results with the jellium model of sodium clusters is presented.

• Received 14 December 1988

DOI:https://doi.org/10.1103/PhysRevLett.62.2301