Alkali halides (MX) are the simplest among the ionic compounds and thus serve as prototypes of ionic bonds. The stability, structure, and charge distribution of alkali halide cluster ions have been interesting subjects because of the ionic characters of the bonds. One of the important issues in the mass spectrum is the magic number. The magic numbers of rare gas clusters have been well studied by using the nuclear shell model and correspond to the shell closing numbers. However, the magic numbers of alkali halide cluster ions are different from those of rare gas clusters because the bonding and the geometrical shape are different from those of rare gas clusters. A simple classical theory of alkali halide cluster ions predicts that the magic numbers in alkali halide cluster ions are related to multiples of the unit cell structure because multiples of the unit cell structure allowed the alkali halide cluster ions to have a maximum coulomb interaction between component ions.

In this work, we recorded the mass spectra of K+(KI)_n, mixed KI/MI cluster ions (M =

Na and Cs), and metal-rich (KI)_n⁺ with a variety of magic numbers.