PHOTOABSORPTION SPECTRA OF SODIUM CLUSTERS

Absolute photoabsorption cross sections of free neutral sodium clusters containing from N = 3 to 40 atoms are presented. Investigation of a wide continuous range of cluster sizes reveals the size development of the photoabsorption behavior. In the smallest clusters the absorption is moleculelike. A transition to collective electronic excitations (surface plasmons) occurs in the size range of N = 3 to 5. For clusters with N = 6 to 12 atoms the surface plasma resonances are particularly well defined, and their positions are consistent with the predictions of an ellipsoidal shell model. The cluster shapes can be deduced from the observed resonance positions; the data provide a sensitive measurement of the relative axis lengths of ellipsoidal clusters. For clusters containing N greater-than-or-similar-to 3 atoms, the plasma resonances do not always coincide with the positions predicted by the ellipsoidal shell model. In addition to these resonances, which dominate the spectra, there are three distinct wavelength regions within which absorption occurs for all investigated clusters.