Identification of the SiF6 2- dianion by accelerator mass spectrometry and a fully relativistic computation of its photodetachment spectrum

The small doubly-charged molecular anion SiF6 2- was studied by two distinct approaches, one experimental the other theoretical. The dianion was produced in the gas phase by sputtering a Li2 SiF6 specimen with Cs+ ions and was detected by means of accelerator mass spectrometry. The identification was via the Si29 F6 2- 19 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 amu) in the mass spectrum, facilitating a positive identification. The flight time through the mass spectrometer of 10 μs establishes a lower limit with respect to the intrinsic lifetime of this species. Attempts to detect the SiF6 2- dianion also by secondary-ion mass spectrometry failed, but provided an upper limit in terms of its formation probability with respect to the F- ion of Si29 F6 2- 19 F-19 <2× 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 SiF6 2- was determined as 2.79 eV at the optimized bond length of 1.718 in the gas phase.