HIGH-RESOLUTION SPECTROSCOPY OF SMALL METAL-CLUSTERS

The optical absorption spectrum of small lithium clusters has been measured up to Li8. In Li3 high resolution Two Photon Ionization (TPI) spectra have been recorded allowing us to determine the geometry and potential surfaces of the ground and excited states. In larger clusters, the excited states are dissociative and the absorption spectra have been obtained by Depletion Spectroscopy. Vibronic resolution is still achieved in Li4, but not in larger clusters. The measured spectra exhibit a rather small number of transitions to electronically excited states. In Li7, only one intense band is observed in the blue region, while in Li8, an intense band is also observed in the blue region and a much weaker band in the red region. All the obtained results are in very good agreement with the ab initio calculation of Bonacic-Koutecky et al. This demonstrates that molecular effects are always present in these small clusters. The semi-classical models of surface plasma resonances are also discussed.