Abstract
The authors discuss the mass spectra intensities of molecular ions CunX+ (X=Cr, Mn, Ni) emitted from a (CuX) alloy bombarded by a primary ion beam. The results for CunCr+ and CunNi+ are taken from the literature. These intensities present a saw-tooth behaviour: they are larger for a given parity of n. According to the correspondence rule 'intensity stability' this phenomenon can be attributed to an alternation of the corresponding stabilities. Then, the authors adopt a quantum mechanical point of view and examine why the stabilities are larger for a given parity of n. In CunX+ ions the X 3d levels keep their atomic d population which is d5 for Cr and Mn, and d9 for Ni (this choice is discussed). The X 4s levels participate in the construction of the s band which is responsible for the stability of the cluster: Cr and Ni bring one electron and one s atomic function, Mn brings two electrons and one s atomic function. According to this simple scheme the s band is a closed shell and, therefore, the cluster is more stable, when the total number of atoms is odd for CunCr+ and CunNi+, and even for CunMn+. This agrees with the secondary ionic emission study (SIMS) results. The authors also discuss the change of Ni behaviour when its proportion in a CunNip+ cluster varies: it appears that the Ni valence configuration is 3d94s only when its proportion is small.
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