UNIMOLECULAR COULOMBIC FISSION IN DOUBLY CHARGED ALKALI-METAL CLUSTERS

Delayed asymmetric fission in two singly charged products is studied for doubly charged potassium clusters K(n)2+ and sodium-potassium clusters NaK(n-1)2+, 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, n(c). 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.