Formation and fragmentation of negative metal clusters - art. no. 023201

We have measured the yield of metal cluster anions of Al, Co, Ni, and Cu pr oduced in a sputter ion source, and also the distribution of negative fragm ent ions after collisions in gases of H-2 and Xe. For Al and Cu the fragmen t distributions show pronounced size effects that can be correlated with th e stability of the species as reflected in the electron affinity. Modeling the collisional energy transfer, electron emission, and evaporation rates, we find that the fragment abundances are strongly influenced by thermal ele ctron emission The abundance spectra of the parent ions from the source sho w a similar correlation with the electron affinity, superimposed on a stron g decrease with cluster size. For Co and Ni the negative-ion yields are muc h smaller than for Al and Cu, both from the source and after fragmentation, and this can he explained by even stronger neutralization by thermal elect ron emission.