Alkali halide cluster dianions: metastability and threshold sizes

Singly and multiply charged salt clusters (MnXn+/-m+/-m; MX = alkali halide ) can be generated simply by spraying saline solutions, with or without ele ctrical (charging) field. Dianion species (m = 2) show a clear threshold si ze at n = 5 - 7 (range) for all elements beyond the first row. These sizes (12 - 16 atoms) are much smaller than previously observed for alkali halide s and are among the smallest cluster dianions yet found in gas phase. We de scribe the experiment and discuss the onsets in terms of ab initio density functional calculations on a number of representative sodium chloride clust er mono- and dianions.