Small silicon-oxygen dianions in the gas phase

Cs+ sputtering of a silicon target with simultaneous exposure of the Si surface to Q(2) 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 Si(n)O2(n+1)(2-) (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. (c) 2006 Elsevier B.V. All rights reserved.