The small doubly-charged molecular anion ${{\mathrm{SiF}}_6}^{2-}$ was studied by two distinct approaches, one experimental the other theoretical. The dianion was produced in the gas phase by sputtering a ${\mathrm{Li}}_2{\mathrm{SiF}}_6$ specimen with ${\mathrm{Cs}}^{+}$ ions and was detected by means of accelerator mass spectrometry. The identification was via the ${}^{29}\mathrm{Si}{}^{19}\mathrm{F}_6^{2-}$ isotopomer; it has an odd total mass and therefore the dianion shows up at a half-integral mass-to-charge ratio $(M∕q=71.5\mathrm{amu})$ in the mass spectrum, facilitating a positive identification. The flight time through the mass spectrometer of $\approx 10\mathrm{\mu }\mathrm{s}$ establishes a lower limit with respect to the intrinsic lifetime of this species. Attempts to detect the ${{\mathrm{SiF}}_6}^{2-}$ dianion also by secondary-ion mass spectrometry failed, but provided an upper limit in terms of its formation probability with respect to the ${\mathrm{F}}^{-}$ ion of ${}^{29}\mathrm{Si}{}^{19}\mathrm{F}_6^{2-}∕{}^{19}\mathrm{F}^{-}<2\times {10}^{-9}$. Furthermore, theoretical calculations of the photoelectron spectrum by means of the relativistic one-particle propagator predict considerable stability of the dianion against autodetachment. The first ionization potential of ${{\mathrm{SiF}}_6}^{2-}$ was determined as $2.79\mathrm{eV}$ at the optimized bond length of $1.718\mathrm{\AA }$ in the gas phase.

• Received 19 March 2008

DOI:https://doi.org/10.1103/PhysRevA.77.053203