Sputtered ion clusters of alkali halides possess magic numbers of N=13

22

etc

which can be well explained by classical electrostatic interactions and quantum 'short-range' interactions. When lithium fluoride crystals of high purity were bombarded by 8keV argon ions

two groups of cluster ions were observed. Both were positively charged

but one of them

[Li (LiF)

N

] +

with microcrystal structure

and the other

(LiF)

N

+

complex ions. And

as the natural lithium has two isotopes

both groups contain great amount of hetero-isotopic clusters. The isotopic effect suggests another important property

in addition to the magic numbers

of mass spectra of the sputtered ion cluster.