Can direct field-evaporation of doubly charged ions and post-ionisation from the singly charged state co-exist?

A tin liquid metal ion source (LMIS) containing ∼14% Pb has been studied. From a careful examination of the mass spectra of the source, it is concluded that post-ionisation (PI) is the most probable mechanism for the formation of Pb²⁺ ions. In the case of tin, however, both Sn ⁺ and Sn²⁺ appear to form by direct field-evaporation. This view is supported by energy deficit measurements on the Sn⁺ and Sn²⁺ beams. Our results for Pb²⁺/Pb⁺ agree with those of Komuro with a pure Pb source. The reason for this is understandable and is related to the similarity between the evaporation fields of Sn and Pb. For the sake of completeness, we also present measurements of the energy spread of the Sn²⁺ and Sn⁺ beams.