Experimental verification of the existence of the gas-phase dianions ${\mathrm{BeF}}_4^{2\mathrm{-}}$ and ${\mathrm{MgF}}_4^{2\mathrm{-}}$

Recent calculations of the electron-detachment energies of the alkaline-earth-metal tetrahalides and alkali-metal trihalides yield values between 1.5 and 3.3 eV. If these values are correct, these molecular ions would be the most stable small dianions of any considered so far, and it should be easy to produce them in negative-ion sources. Using a cesium sputter source and accelerator mass spectrometry (AMS), we have detected and identified two multiply charged alkaline-earth-metal molecular-cluster ions, ${\mathrm{BeF}}_4^{2\mathrm{-}}$ and ${\mathrm{MgF}}_4^{2\mathrm{-}}.$ Using the techniques that we developed earlier to study ${\mathrm{C}}_n^{2-},$ we have been able to use AMS to unambiguously identify these dianions. We provide further evidence for their existence in the form of Coulomb explosion images that we produced in the terminal of our tandem accelerator. An unsuccessful search for alkali-metal dianions of the form ${\mathrm{MX}}_3^{2-}$ and $M_2{X}_4^{2-}$ $({\mathrm{LiF}}_3^{2\mathrm{-}}$ and ${\mathrm{Li}}_2{\mathrm{F}}_4^{2\mathrm{-}})$ allows us to set an upper limit of ${10}^{-17}$ relative to ${\mathrm{F}}^{\mathrm{-}}$ for their production in a Cs sputter source assuming their lifetimes are greater than 10 μs. It seems likely that they are either not formed at all in the sputter source or their lifetimes are considerably shorter than 10 μs.