Production of small doubly charged negative carbon cluster ions by sputtering

Carbon cluster ions with a double negative charge, and lifetimes of at least 10 μs, have been produced by sputtering graphite, and observed in a double-focusing mass spectrometer. Ions as small as ${\mathrm{C}}_7^{2\mathrm{-}}$ have been detected. The intensities of the doubly charged clusters (${\mathrm{C}}_{\mathit{n}}^{2\mathrm{-}}$ where n=7–28) follow an alternating pattern, with even-n clusters being more intense than neighboring odd-n clusters. In contrast, even-n singly charged clusters are more intense than odd n for n≤9, and less intense for n≥13, where the clusters are believed to be cyclic.