IONIZATION AND FRAGMENTATION OF METALLIC CLUSTERS (NA-N) INDUCED BY ION IMPACT

Neutral sodium clusters (Na-n, n less than or equal to 800) have been ionized in collisions with different ion beams (H+, O5+, Ar8+) at collision energies between 1 and 19 keV per charge. From an analysis of the measured mass/charge spectrum critical sizes for multiply charged clusters are deduced and compared with those obtained by laser-ionization. The dominant ionization mechanism depends on the charge state of the projectile. For highly charged ions the multi-electron capture process leads to the formation of relatively ''cold'' multiply charged clusters; in the proton case multiple ionization is discussed in terms of electronic excitation followed by electron evaporation on a sub-nanosecond time scale.