Mass distributions of aluminum negative cluster ions and their binary cluster ions mixed with a carbon atom

The mass distributions of aluminum negative cluster ions and their binary cluster ions mixed with a carbon atom, generated by laser vaporization, were measured. In the range from n=5 to 70, the mass distribution of pure aluminum negative cluster ions, Al^-_n, exhibits one peak at n = 13 and one step between n=23 and 24. The mass distribution of the series of Al_nC^-, however, exhibits one peak at n=7 and one step between n=24 and 25. The absence of a peak in the series of Al_nC^- around n=13 can be explained by the electronic structure in relation to the electronic shell model, and both the peak and the step in the mass distribution of Al_nC^- clusters can be explained by their geometric structure.