The anions of the Li, Be and B fluorides: The super-halogens and AMS

During tests with BeF2 samples in a sputter ion source, it was found that the largest negative fluoride ion current was of BeF3-. This is because BeF3 belong to an extensive class of molecules known as super-halogens. These have electron affinities much higher than those of the halogens themselves and so are of great potential interest to AMS. A systematic study of anions of this class for Li, Be and B was therefore undertaken. The strongest fluoride anions measured were LiF2-, BeF3- and BF4- as expected. The anion of LiF3 could not be observed at a ratio of LiF3-/LiF2-∼10-8. The anion of BF3, which is predicted like the anion of BF to be unstable, was observed with intensity modestly greater than that of the BF anion. These measurements illustrate some general properties of super-halogen anions: (1) their great electron binding energies make it possible to produce large currents with Cs+ sputter sources; and (2) the super-halogen anions of the neighbouring elements are often one halogen atom apart, so some degree of isobar selectivity is available at the ion source. The possibility of directly counting 7Be2+ at <2 MV terminal voltage, by simply injecting BeF3-, is an example of this realization. Other ramifications of super-halogen anions for low energy AMS development are also discussed.