An experimental setup is described, which is used to multi-ionize neutral clusters and to study their stability and the importance of different decay processes. Clusters are ionized in collisions with slow multiply charged ions (projectile charge z ranging from 1 to 30, kinetic energies ranging from 1 to 20 keV/charge). Both ion and cluster beamlines, as well as the characteristics of the analysis and detection systems, are described. Collisions with highly charged ions such as Ar8+ or Xe30+ turn out to be efficient tools in preparing clusters in high charge states without increasing significantly their internal temperature. Measurements performed in coincidence with the number of electrons stabilized by the projectile ion after the collision allowed us to control the charge and the excitation energy of the ionized system. The efficiency of the method is demonstrated for sodium clusters and C60 molecules.

Fission of multiply charged sodium clusters Na n qϩ produced in collisions of neutral clusters with protons at 20 keV is investigated with an event-by-event acquisition technique. The analysis of fragment correlations shows that clusters undergo a strongly asymmetrical binary fission with Na ϩ , Na 2 ϩ , Na 3 ϩ , Na 4 ϩ , and Na 5 ϩ as the small fragments, Na 3 ϩ dominating with approximately 70%. For a given charge q, the branching ratio of Na 3 ϩ over Na ϩ emission increases with n, which is interpreted in terms of a decreasing temperature of the parent cluster following proton impact.Charged alkali-metal clusters become unstable when the long-range Coulomb repulsion due to the net positive electric charge overcomes the electronic binding. This instability is analogous to that prevailing in heavy nuclei. Nuclear fission involves a large amplitude collective motion resulting in a more or less symmetric binary fragmentation. Symmetric fission of heavy nuclei is expected from the nuclear liquid-drop model ͓1͔. Within this model, metal clusters are different from atomic nuclei because they are highly polarizable conducting droplets and not homogeneously charged ones. In the absence of mass to charge coupling ͑as for neutrons and protons in nuclei͒, one may expect a charge splitting that is different from the mass splitting. In any case, the equilibrium shape for a neutral drop is spherical. For a charged drop, the dependence of both the surface and the Coulomb energy upon small deformations is such that infinitesimal quadrupole fluctuations trigger a shape instability once the ratio X of the Coulomb energy to twice the surface energy exceeds the Rayleigh limit Xϭ1 ͓2͔. This ratio, called the fissility parameter, is proportional to q 2 /n:where q is the net electric charge and n is the number of constituents of the cluster. The factor ␣ is about 0.02 for nuclear matter, whereas it is about 2.5 for sodium ͑of interest in this paper͒. Treating sodium clusters as liquid drops, one thus expects the critical sizes for stability to be about 10,23,40,63, . . . for charges qϭ2,3,4,5, . . . , respectively. These critical sizes are hard to reach experimentally because it requires forming multiply charged sodium clusters at zero temperature. Otherwise fission can be activated thermally.There have been essentially two experimental methods used so far to prepare multiply charged clusters and to study their instability. The first one uses a multistep photoionization technique ͓3͔. Due to the photon energy, there is a sizedependent limit for obtaining multiply charged clusters. Moreover, multiply charged clusters produced this way turn out to be stable towards fission. To obtain fission, it is necessary to evaporate a substantial fraction of atoms until the cluster size has reached the size for which the height of the barrier against charge splitting becomes smaller than the dis-sociation energy for evaporation, 1 eV for sodium. Thus the measured appearance sizes n app (q) that are defined as the minimum size of q-fold clusters ap...

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