Surface plasma resonances in free metal clusters

Photoabsorption cross sections of small free neutral sodium clusters are measured in the wavelength range of 452--604 nm. The results indicate that the photoabsorption spectra are dominated by surface plasma oscillations of the valence electrons. The measured photoabsorption cross sections are consistent with a sum-rule calculation based on an ellipsoidal shell model and the experimental static electric polarizabilities of Na clusters. The calculated photoabsorption spectra of closed-shell clusters contain a single surface-plasma-resonance peak, whereas those of open-shell clusters have double or triple peaks. The experimental magnitudes of the photoabsorption cross sections near resonance peaks are between 1 and 2 A² per delocalized electron.