Energy-resolved collision-induced dissociation of Al-n(+) clusters (n=2-11) in the center of mass energy range from few hundred meV to 10 eV

Energy-resolved collision-induced dissociation (CID) of Al-n(+) (n = 2 - 11 ) in collision with argon is presented for the energy ranges from few hundr ed meV to 10 eV in the center of mass frame. The experiments were carried o ut with a recently constructed secondary ion tandem mass spectrometer, that is described in detail. The collision energy dependence is measured for th e total and the partial dissociation cross sections, and the dissociation t hresholds for the individual processes are estimated. The release of Al+ is found to be the dominating channel for n < 8. For n > 8, the cross section for the release of Al+ and Al are comparable. The release of more than one neutral atom from the larger clusters (n > 6) is found to be in good agree ment with sequential atom loss. In the case of the smaller clusters, on the other hand, fission is the energetically favorable process. The closed she ll cluster, Al-7(+) (20 valence electrons), is found to be exceptionally st able and the adiabatic ionization potential of Al-7 is found to be lower th an that of the monomer. The stability of Al-7(+) is further reflected in th e dissociation dynamics of the next neighbor, Al-8(+). The high stability o f Al-7(+) as well as the dissociation dynamics of Al-8(+) are treated in th e simple frame of the electronic shell model. Unlike Al-7(+), Al-3(+) (with 8 valence electrons) shows no sign of increased stability, and the dissoci ation dynamics seems to be controlled by the spin selection rules, rather t han the energetics. In the present work, general trends and the dissociatio n dynamics of individual clusters are discussed. Qualitative information on the development of the geometric and electronic structure, with increasing cluster size, is deduced and discussed in terms of a transition from a cov alent to a metallic character. Finally, this work is compared to earlier th eoretical and experimental approaches to Al-n(+) clusters. (C) 1999 America n Institute of Physics. [S0021-9606(99)00808- 9].