Photoabsorption cross sections of small neutral sodium clusters composed of N=2–40 sodium atoms are measured by longitudinal–beam-depletion spectroscopy at several wavelengths of visible light. Absorption occurs via coupling of photons to collective oscillations of the valence electrons. The cross section is strongly size and wavelength dependent. Good agreement is found with predictions based on an extended Clemenger-Nilsson shell model and the experimental static polarizabilities.

• Received 20 August 1987

DOI:https://doi.org/10.1103/PhysRevLett.59.1805