Metal-cluster fission and the liquid-drop model

Measurements of the fission of multiply charged Au clusters are presented and interpreted in the context of the liquid-drop model. A detailed description of the experimental apparatus is given, with particular attention paid to the cluster source, a liquid-metal ion source. It is found that upon excitation by either photons or collisions, doubly charged clusters in the size range n=12--18 atoms may either evaporate a neutral atom or undergo fission, with the tendency towards fission increasing as cluster size decreases. Empirically the relative rate for fission is found to depend on z²/n. This result is shown to be consistent with the liquid-drop model developed for the fission of heavy nuclei [N. Bohr and J. A. Wheeler, Phys. Rev. 57, 426 (1939)]. The fission barrier height, calculated from the Coulomb and surface energies of a deformed, charged metallic droplet, decreases strongly with cluster size, in agreement with the experimental results.