Sputtered gas-phase dianions detected by high-sensitivity mass spectrometry

The detection of small doubly-charged molecular anions by means of highly sensitive mass spectrometry is discussed. The production of these gas-phase dianions is accomplished by sputtering the specimen with Cs+ ions with an energy of a few keV. It is demonstrated that dianions can be detected most easily when the molecular ion has an odd total mass; then, the dianions will show up at half-integral mass numbers in the mass spectrum. In addition, the agreement of the relative abundances of several isotopomers of a dianion with the nominal isotopic pattern corroborates the identification of a dianionic species in the mass spectrum. These features are exemplified by monitoring mixed silicon-oxygen dianions of the general form SinO2n+12- (with 2 ⩽ n ⩽ 8) in a low-energy mass spectrometer. They were formed by sputtering a silicon wafer at an elevated oxygen partial pressure in the vicinity of the sample's surface. The flight time through the mass spectrometer of ∼15-30 μs establishes a lower limit with respect to the intrinsic lifetimes of these dianions. Emission energy spectra of various singly- and doubly-charged ions illustrate the occurrence of fragmentation processes. The yields of the doubly- and singly-charged mixed silicon-oxygen anions increase with the ratio of the O2 arrival rate to the Cs+ flux density, but tend to saturate when this ratio approaches unity. The benefits of high-energy accelerator mass spectrometry in dianion detection are illustrated for LiF32- and CaF42-.