Formation and fragmentation of negative metal clusters

Abstract

We have measured the yield of metal cluster anions of Al, Co, Ni, and Cu produced in a sputter ion source, and also the distribution of negative fragment ions after collisions in gases of ${\mathrm{H}}_{2}$ and Xe. For Al and Cu the fragment distributions show pronounced size effects that can be correlated with the 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 electron emission. The abundance spectra of the parent ions from the source show a similar correlation with the electron affinity, superimposed on a strong decrease with cluster size. For Co and Ni the negative-ion yields are much smaller than for Al and Cu, both from the source and after fragmentation, and this can be explained by even stronger neutralization by thermal electron emission.