Formation of gas-phase oxygen-carbon OC 2- n dianions by sputtering

The first small doubly charged negative cluster ions OC 2 - n (with n = 7-14) 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. These dianions and the corresponding singly charged OC n - (n≤21) cluster ions were detected in a double-focusing mass spectrometer. The yields of doubly and of singly charged mixed oxygen-carbon ions increase with the ratio of the O 2 arrival rate to the Cs + flux density; they tend to saturate with this ratio approaching unity. For n≥11, the ion yields of the OC n 2- dianions are comparable to those of the singly charged OC n - ions. For both types of ion species, the abundance distributions exhibit even-odd alternations with the number of C atoms in the cluster. The flight time through the mass spectrometer of ∼15-20 μs establishes a lower limit with respect to the intrinsic lifetimes of the doubly charged ions.