Unimolecular Coulombic fission in doubly charged alkali-metal clusters

Delayed asymmetric fission in two singly charged products is studied for doubly charged potassium clusters Kₙ²⁺ and sodium-potassium clusters NaK_n-1²⁺, in the size range n=19--28. The small size limit coincides with the limit of the observability of doubly charged clusters in mass spectra, the so-called critical size of stability, nc. Unimolecular-dissociation experiments probe the competition between the Coulombic fission and the neutral-monomer evaporation around the critical size. When they exist, fission channels mainly include the most stable trimer ion. The metallic-drop model including shell effects well reproduces the experimental unimolecular-dissociation channels. Moreover, it shows that the asymmetric character of the Coulombic fission is mainly due to the surface tension of the cluster.