Discovery of a new class of stable gas-phase dianions: Mixed oxygen–carbon cluster OCn2− (n=5–19)

Abstract

Small doubly charged negative cluster ions OCn2− were studied both by experimental and by theoretical means. In the experiments these dianions (with n=5–19) were produced by sputtering of a graphite specimen with a 14.5 keV Cs+ ion beam at an elevated oxygen partial pressure in the vicinity of the sample’s surface. The dianions and the corresponding singly charged OCn− (n⩽21) ions as well as homonuclear carbon dianions Cn2− were detected in a double-focusing mass spectrometer. The yields of the doubly and singly charged mixed oxygen-carbon ions increase with the ratio of the O2 arrival rate to the Cs+ flux density. The abundance distribution of OCn2− exhibits distinct even–odd alternations with the number of C atoms in the molecule. The flight time through the mass spectrometer of ∼15–25 μs establishes a lower limit with respect to the intrinsic lifetimes of the doubly charged ions. The theoretical studies investigated OCn2− dianions with n=5–8; the geometries of these species were optimized and the electronic stability was examined by the calculation of the electron detachment energies. In agreement with the low yield observed experimentally, OC52− is found to be weakly stable, whereas OC62−, OC72−, and OC82− are electronically stable gas-phase ions. In particular the latter is characterized by a large number of stable isomers.