Doubly charged negative silicon-carbon clusters produced in sputtering

Small doubly charged negative cluster ions ${\mathrm{SiC}}_n^{2-}$ (with $n=6,$ 8, and 10) are produced by sputtering the surface of a SiC specimen with a 14.5-keV ${\mathrm{Cs}}^{+}$-ion beam. They are detected in a double-focusing mass spectrometer that covers a dynamic abundance range of about ${10}^9.$ The emission yields of these dianionic clusters amount to roughly ${10}^{-4}$ of the corresponding singly charged cluster ions. For both types of ion species, the abundance distributions decrease monotonically with an increasing number of C atoms in the cluster. This observation is ascribed to fragmentation processes that are due to the high amount of internal energy relayed to the cluster species in the sputtering event. Apart from this decomposition caused by excitation, the flight time through the mass spectrometer of ∼15 μs establishes a lower limit with respect to the intrinsic lifetimes of both the singly and doubly charged ions.