Collision induced dissociation of metal cluster ions: Bare aluminum clusters, Al+n (n=3–26)

Abstract

The collision‐induced dissociation of aluminum clusters, Al⁺_n (n=3–26), by argon, at a center of mass collision energy of 5.25 eV, has been studied using a low energy ion beam apparatus. Product branching ratios and collision induced dissociation cross sections are presented and discussed. The main product is Al⁺ for the smaller clusters and Al⁺_n−1 for the larger ones. The cross sections rise to a peak at Al⁺₆−Al⁺₉ and then decrease with increasing cluster size. Cross sections for Al⁺₇, Al⁺₁₃, Al⁺₁₄, and Al⁺₂₃ are significantly smaller than their neighbors. A crude kinetic model is used to derive approximate cluster ionization potentials from the product branching ratios. The IPs initially rise with cluster size, peak at Al₆ and then decrease. The IP of Al₇ is particularly low and there is a sharp drop in IP at Al₁₄ where the IP falls below that of the atom. The results suggest that the dissociation energies increase for the larger clusters and there is evidence that Al⁺₇, Al₁₃, Al⁺₁₃, Al⁺₁₄, and Al⁺₂₃ have enhanced stability. The results are compared to the predictions of the electronic shell model which can account for some of the results but predicts additional features which are not observed.