Small silicon-oxygen dianions in the gas phase

Cs+ sputtering of a silicon target with simultaneous exposure of the Si surface to O2 gas has produced small silicon-oxygen dianions SinO2n+12- (2 ⩽ n ⩽ 8) as well as Si5O102-. These dianions were observed by mass spectrometry for ion flight times of a few 10−5 s. The two smallest species, Si2O52- and Si3O72-, have been identified unambiguously by their isotopic abundances. Ab initio electronic structure calculations are used to study the geometrical structure of the three most abundant species SinO2n+12- (n = 3, 4, 5), and for all three dianions several stable isomers are found. The structural motifs of the low-energy isomers are analyzed, and include in particular structures with trivalent oxygen atoms akin to impurities in solid silicon.