DOUBLY-CHARGED SODIUM CLUSTERS - PLASMON RESPONSE AND PHOTOPRODUCTION

The optical response of doubly charged sodium clusters Na-n+2(++) was measured for n = 20, 40, and 58 valence electrons, for which the jelli um model predicts spherical clusters. A new experimental scheme was de veloped which allows to separate doubly charged clusters of even mass from the singly charged with half the mass. The optical spectra are do minated by a plasmon-like resonance which is blue shifted and narrower than that of the singly charged clusters. The smallest doubly charged cluster observed was Na-9(++). The photo ionization cross section for singly charged clusters was found to be typically 2.6 . 10(-19) cm(2) per Na atom for photon energies of around 6 eV, which is a factor of 400 smaller than the maximum in the plasmon absorption in the region o f (h) over bar omega=2.6 eV.