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 electrical (charging) field. Dianion species (m = 2) show a clear threshold size 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 halides and are among the smallest cluster dianions yet found in gas phase. We describe the experiment and discuss the onsets in terms of ab initio density functional calculations on a number of representative sodium chloride cluster mono- and dianions.