Aluminum-boron negative cluster ions (AlnB- m, n=5-30, m=1-3) were generated by two independent laser-vaporization sources. The mass distribution of AlnB- 2 exhibits one peak at n=12 and one step between n=22 and 23 as magic numbers, whereas that of AlnB- 2 exhibits only one step between n=21 and 22. The loss of the magic number of Al11B- 2 can be explained by the geometric structures: Al12B- 1 is stabilized by both an electronic shell closing of the 2p shell and a close-packing structure of icosahedron. In contrast, Al11B- 2 can satisfy the electronic shell closing by cannot take the stable geometric structure because the diameters of Al and B atoms differ largely. The structure of Al12B- 1 can be an icosahedron with a B atom located at the center.
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