Asymmetric fission ofNan++around the critical size of stability

Unimolecular dissociation channels of mass-selected doubly charged sodium clusters ${\mathrm{Na}}_{\mathit{n}}^{++}$ have been determined for cluster sizes near and above the critical size, ${\mathit{n}}_{\mathit{c}}$=27, from which doubly charged clusters are detectable in mass spectra. On a time scale of 50 μs with respect to ionization, doubly charged clusters of size n=${\mathit{n}}_{\mathit{c}}$ are found to fission asymmetrically. For cluster sizes above ${\mathit{n}}_{\mathit{c}}$, however, evaporation of a neutral monomer is the dominant channel for delayed dissociation. An estimation of the height of the Coulombic barrier is given for ${\mathrm{Na}}_{\mathit{n}}^{++}$ near the critical size, using the framework of statistical theory.