MULTIPLE IONIZATION OF SODIUM CLUSTERS BY ION IMPACT

The electronic excitation of alkali-metal clusters by low-energy ions is investigated experimentally for the first time. Multi-ionized clust ers possibly undergoing Coulomb dissociation are formed and detected. Free sodium clusters of a few hundred atoms have been bombarded by dif ferent ion beams (H+, O5+, Ar8+) of velocity ranging from 0.2 to 0.9v( 0). The mass/charge ratio of charged outgoing clusters is measured by a time-of-flight spectrometer of high resolution (m/Delta m = 2500). C ritical sizes for stability against charge excess have been deduced fo r cluster charges up to 6. Temperature effects on critical sizes are o bserved, and they reveal two distinct regimes: electron transfer in pe ripheral collisions with multicharged ions leading to ''cold'' highly ionized clusters and strong electronic excitation in central collision s subsequently followed by multielectron emission.