Beams of doubly ionized molecules from a tandem accelerator

We report the observation of 3-4 MeV beams of several species of small doubly charged molecules (¹⁰B¹¹B⁺⁺, ¹⁰B₂⁺⁺, ¹¹B¹²C ⁺⁺, ⁹Be¹⁴N⁺⁺, ¹²C ¹³C⁺⁺, ¹²C¹³CH⁺⁺, ¹²C¹⁴N⁺⁺, ¹²C²H ⁺⁺, ¹²CH₂⁺⁺, ⁹Be ¹⁶O⁺⁺, ¹⁰B¹⁶O⁺⁺) emerging from a tandem accelerator. These observations significantly increase the number of such molecules known. All are probably produced in metastable states with lifetimes longer than 1 μs. Substantial fluxes (> 10⁸ molecules per second was shown for CN⁺⁺) of these doubly charged molecules were obtained and they were identified by observing the fragmentation of molecular ions after passing through a carbon foil. In some cases the molecules are observed to decay in flight. An efficiency of 10^-4 in producing mass 25 molecules (¹²C¹³C⁺⁺ + ¹²C₂H⁺⁺) has been measured. Advantage is taken of the existence of "atomic interference free" mass regions, where no atomic negative ions are formed, therefore allowing the production of pure molecular beams. Some implications of this work for accelerator mass spectrometry (AMS) are discussed. An application of doubly positive charged molecules in a search for doubly negative charged molecules was made, establishing an upper limit of 1:10¹⁰ of ¹⁰B ¹¹B^--/¹⁰B¹¹B^- and of 1:10⁹ of ¹²C¹³C^--/ ¹²C¹³C^-.