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 A${̊}^2$ per delocalized electron.